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Secondary Lymphatic Organs

The secondary lymphoid organs Lymphoid organs A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs, also called peripheral lymphoid/lymphatic organs, include MALT MALT Colon, Cecum, and Appendix: Anatomy, the lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 - 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy, and the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy. These collections of lymphoid tissues provide constant surveillance Surveillance Developmental Milestones and Normal Growth for pathogens. In their distinct sites, these tissues are filled with immune cells ready to mount a response when antigens are detected. Lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 - 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy are seen along lymphatic vessels Lymphatic Vessels Tubular vessels that are involved in the transport of lymph and lymphocytes. Lymphatic Drainage System: Anatomy and occur as chains or groups ( neck Neck The part of a human or animal body connecting the head to the rest of the body. Peritonsillar Abscess, groin Groin The external junctural region between the lower part of the abdomen and the thigh. Male Genitourinary Examination, axillae, mesenteries, abdomen). MALTs act as immune sensors situated in locations where the lumen is exposed to the external environment ( oropharynx Oropharynx The middle portion of the pharynx that lies posterior to the mouth, inferior to the soft palate, and superior to the base of the tongue and epiglottis. It has a digestive function as food passes from the mouth into the oropharynx before entering esophagus. Pharynx: Anatomy, GI tract, genitourinary tract). The spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy is the site of production of antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions and lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology, but it also aids AIDS Chronic HIV infection and depletion of CD4 cells eventually results in acquired immunodeficiency syndrome (AIDS), which can be diagnosed by the presence of certain opportunistic diseases called AIDS-defining conditions. These conditions include a wide spectrum of bacterial, viral, fungal, and parasitic infections as well as several malignancies and generalized conditions. HIV Infection and AIDS in the breakdown of platelets Platelets Platelets are small cell fragments involved in hemostasis. Thrombopoiesis takes place primarily in the bone marrow through a series of cell differentiation and is influenced by several cytokines. Platelets are formed after fragmentation of the megakaryocyte cytoplasm. Platelets: Histology and erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology.

Last updated: 9 Mar, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Overview

Immune system Immune system The body’s defense mechanism against foreign organisms or substances and deviant native cells. It includes the humoral immune response and the cell-mediated response and consists of a complex of interrelated cellular, molecular, and genetic components. Primary Lymphatic Organs

  • The immune system Immune system The body’s defense mechanism against foreign organisms or substances and deviant native cells. It includes the humoral immune response and the cell-mediated response and consists of a complex of interrelated cellular, molecular, and genetic components. Primary Lymphatic Organs provides defense (immunity) against invading pathogens ranging from viruses Viruses Minute infectious agents whose genomes are composed of DNA or RNA, but not both. They are characterized by a lack of independent metabolism and the inability to replicate outside living host cells. Virology to parasites. The components of the immune system Immune system The body’s defense mechanism against foreign organisms or substances and deviant native cells. It includes the humoral immune response and the cell-mediated response and consists of a complex of interrelated cellular, molecular, and genetic components. Primary Lymphatic Organs are interconnected by blood and the lymphatic circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment.
  • 2 overlapping lines of defense Lines of Defense Inflammation:
    • Innate immunity Innate immunity The capacity of a normal organism to remain unaffected by microorganisms and their toxins. It results from the presence of naturally occurring anti-infective agents, constitutional factors such as body temperature and immediate acting immune cells such as natural killer cells. Innate Immunity: Phagocytes and Antigen Presentation (nonspecific), involving the following cells:
      • Natural killer cells Natural killer cells A specialized subset of T-lymphocytes that exhibit features of innate immunity similar to that of natural killer cells. They are reactive to glycolipids presented in the context of the major histocompatibility complex (MHC) class I-like molecule, CD1D antigen. Lymphocytes: Histology
      • Monocytes Monocytes Large, phagocytic mononuclear leukocytes produced in the vertebrate bone marrow and released into the blood; contain a large, oval or somewhat indented nucleus surrounded by voluminous cytoplasm and numerous organelles. Innate Immunity: Phagocytes and Antigen Presentation/ macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation
      • Dendritic cells Dendritic cells Specialized cells of the hematopoietic system that have branch-like extensions. They are found throughout the lymphatic system, and in non-lymphoid tissues such as skin and the epithelia of the intestinal, respiratory, and reproductive tracts. They trap and process antigens, and present them to T-cells, thereby stimulating cell-mediated immunity. They are different from the non-hematopoietic follicular dendritic cells, which have a similar morphology and immune system function, but with respect to humoral immunity (antibody production). Skin: Structure and Functions
      • Neutrophils Neutrophils Granular leukocytes having a nucleus with three to five lobes connected by slender threads of chromatin, and cytoplasm containing fine inconspicuous granules and stainable by neutral dyes. Innate Immunity: Phagocytes and Antigen Presentation, basophils Basophils Granular leukocytes characterized by a relatively pale-staining, lobate nucleus and cytoplasm containing coarse dark-staining granules of variable size and stainable by basic dyes. Innate Immunity: Phagocytes and Antigen Presentation, eosinophils Eosinophils Granular leukocytes with a nucleus that usually has two lobes connected by a slender thread of chromatin, and cytoplasm containing coarse, round granules that are uniform in size and stainable by eosin. Innate Immunity: Phagocytes and Antigen Presentation
      • Tissue mast cells Mast cells Granulated cells that are found in almost all tissues, most abundantly in the skin and the gastrointestinal tract. Like the basophils, mast cells contain large amounts of histamine and heparin. Unlike basophils, mast cells normally remain in the tissues and do not circulate in the blood. Mast cells, derived from the bone marrow stem cells, are regulated by the stem cell factor. Innate Immunity: Phagocytes and Antigen Presentation
    • Adaptive immunity (based on specific antigen Antigen Substances that are recognized by the immune system and induce an immune reaction. Vaccination recognition) involves the following cells:
      • T and B lymphocytes B lymphocytes Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions (derived from lymphoid organs Lymphoid organs A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs)
      • Antigen-presenting cells Antigen-presenting cells A heterogeneous group of immunocompetent cells that mediate the cellular immune response by processing and presenting antigens to the T-cells. Traditional antigen-presenting cells include macrophages; dendritic cells; langerhans cells; and B-lymphocytes. Follicular dendritic cells are not traditional antigen-presenting cells, but because they hold antigen on their cell surface in the form of immune complexes for b-cell recognition they are considered so by some authors. Adaptive Immune Response

Lymphatic system Lymphatic system A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs

The lymphatic system Lymphatic system A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs ( lymph vessels Lymph vessels Tubular vessels that are involved in the transport of lymph and lymphocytes. Lymphatic Drainage System: Anatomy, lymph fluid Lymph fluid The interstitial fluid that is in the lymphatic system. Lymphatic Drainage System: Anatomy, and lymphoid organs Lymphoid organs A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs) is part of the body’s immune system Immune system The body’s defense mechanism against foreign organisms or substances and deviant native cells. It includes the humoral immune response and the cell-mediated response and consists of a complex of interrelated cellular, molecular, and genetic components. Primary Lymphatic Organs.

  • Lymphoid organs Lymphoid organs A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs:
    • Primary: 
      • Locations where lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology develop from progenitor cells (initial formation)
      • Include the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types marrow and thymus Thymus A single, unpaired primary lymphoid organ situated in the mediastinum, extending superiorly into the neck to the lower edge of the thyroid gland and inferiorly to the fourth costal cartilage. It is necessary for normal development of immunologic function early in life. By puberty, it begins to involute and much of the tissue is replaced by fat. Lymphatic Drainage System: Anatomy
    • Secondary: 
      • Sites where lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology undergo activation, proliferation, and additional maturation
      • Include the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy, lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy, and MALT MALT Colon, Cecum, and Appendix: Anatomy (i.e., tonsils Tonsils Tonsillitis)
  • Lymphatic vessels Lymphatic Vessels Tubular vessels that are involved in the transport of lymph and lymphocytes. Lymphatic Drainage System: Anatomy and ducts:
Anatomy of the lymphatic system

Anatomy of the lymphatic system Lymphatic system A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs:
Includes the primary ( bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types marrow, thymus Thymus A single, unpaired primary lymphoid organ situated in the mediastinum, extending superiorly into the neck to the lower edge of the thyroid gland and inferiorly to the fourth costal cartilage. It is necessary for normal development of immunologic function early in life. By puberty, it begins to involute and much of the tissue is replaced by fat. Lymphatic Drainage System: Anatomy) and secondary ( spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy, lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy, and MALT MALT Colon, Cecum, and Appendix: Anatomy) lymphoid organs Lymphoid organs A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs
Lymphatic vessels Lymphatic Vessels Tubular vessels that are involved in the transport of lymph and lymphocytes. Lymphatic Drainage System: Anatomy convey lymph to the larger lymphatic vessels Lymphatic Vessels Tubular vessels that are involved in the transport of lymph and lymphocytes. Lymphatic Drainage System: Anatomy in the torso, transporting fluid back to the venous circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment.

Image: “Anatomy of the Lymphatic System Lymphatic system A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs” by OpenStax. License: CC BY 4.0

Lymph Nodes

Anatomy

Lymph nodes and the lymphatic system

Lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy and the lymphatic system Lymphatic system A system of organs and tissues that process and transport immune cells and lymph. Primary Lymphatic Organs

Image: “Blausen 0623 LymphaticSystem Female” by Blausen. License: CC BY 3.0

Main functions

The main function of lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy is defense against the spread of microorganisms and tumor Tumor Inflammation cells:

  • Filtration of lymph
  • Production and recirculation of lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology
  • Development of plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products cells secreting antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions
  • Provide closed environments for antigen Antigen Substances that are recognized by the immune system and induce an immune reaction. Vaccination presentation Presentation The position or orientation of the fetus at near term or during obstetric labor, determined by its relation to the spine of the mother and the birth canal. The normal position is a vertical, cephalic presentation with the fetal vertex flexed on the neck. Normal and Abnormal Labor

General structure

  • Outer capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides ( dense connective tissue Dense connective tissue Connective Tissue: Histology):
    • Afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology lymphatic vessels Lymphatic Vessels Tubular vessels that are involved in the transport of lymph and lymphocytes. Lymphatic Drainage System: Anatomy penetrate the capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides
    • Trabeculae extend from the outer capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides toward the center of nodules.
    • Blood vessels penetrate through trabeculae.
  • Within the capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides:
    • Cells:
      • Lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology
      • Plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products cells
      • Dendritic cells Dendritic cells Specialized cells of the hematopoietic system that have branch-like extensions. They are found throughout the lymphatic system, and in non-lymphoid tissues such as skin and the epithelia of the intestinal, respiratory, and reproductive tracts. They trap and process antigens, and present them to T-cells, thereby stimulating cell-mediated immunity. They are different from the non-hematopoietic follicular dendritic cells, which have a similar morphology and immune system function, but with respect to humoral immunity (antibody production). Skin: Structure and Functions
      • Macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation
      • Other antigen-presenting cells Antigen-presenting cells A heterogeneous group of immunocompetent cells that mediate the cellular immune response by processing and presenting antigens to the T-cells. Traditional antigen-presenting cells include macrophages; dendritic cells; langerhans cells; and B-lymphocytes. Follicular dendritic cells are not traditional antigen-presenting cells, but because they hold antigen on their cell surface in the form of immune complexes for b-cell recognition they are considered so by some authors. Adaptive Immune Response
      • Follicular dendritic cells Follicular dendritic cells Non-hematopoietic cells, with extensive dendritic processes, found in the primary and secondary follicles of lymphoid tissue (the B cell zones). They are different from conventional dendritic cells associated with T-cells. They are derived from mesenchymal stem cells and are negative for class II mhc antigen and do not process or present antigen like the conventional dendritic cells do. Instead, follicular dendritic cells have fc receptors and C3b receptors that hold antigen in the form of antigen-antibody complexes on their surfaces for long periods for recognition by B-cells. MALT Lymphoma 
    • Regions are formed by the cellular arrangements in the stroma of reticulin fibers and reticular cells: cortex, paracortex, and medulla.
Histologic section of a lymph node identifying the capsule and the trabeculae

Histologic section of a lymph node, identifying the capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides and the trabeculae

Image by Geoffrey Meyer, edited by Lecturio.

Functional regions

  • Cortex: 
    • Subcapsular sinus:
      • Structure immediately inside the capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides
      • Cortical sinuses (or trabecular sinuses) penetrate and branch internally along trabeculae. 
      • Through these sinuses, lymph (with antigens, antigen-presenting cells Antigen-presenting cells A heterogeneous group of immunocompetent cells that mediate the cellular immune response by processing and presenting antigens to the T-cells. Traditional antigen-presenting cells include macrophages; dendritic cells; langerhans cells; and B-lymphocytes. Follicular dendritic cells are not traditional antigen-presenting cells, but because they hold antigen on their cell surface in the form of immune complexes for b-cell recognition they are considered so by some authors. Adaptive Immune Response, and lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology) passes and seeps into the lymphoid tissue.
    • Lymphoid nodules (follicles):
      • Primary follicles (inactive): highly packed B cells B cells Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions in reticular fibers
      • Secondary follicles (active): have pale germinal centers (site of B-cell proliferation; contains lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology, lymphoblasts Lymphoblasts Lymphocytosis, and follicular dendritic cells Follicular dendritic cells Non-hematopoietic cells, with extensive dendritic processes, found in the primary and secondary follicles of lymphoid tissue (the B cell zones). They are different from conventional dendritic cells associated with T-cells. They are derived from mesenchymal stem cells and are negative for class II mhc antigen and do not process or present antigen like the conventional dendritic cells do. Instead, follicular dendritic cells have fc receptors and C3b receptors that hold antigen in the form of antigen-antibody complexes on their surfaces for long periods for recognition by B-cells. MALT Lymphoma)
  • Paracortex:
    • T-cell zone
    • Region inside the cortex
    • No obvious boundaries but lacks follicles
    • High endothelial venules Venules The minute vessels that collect blood from the capillary plexuses and join together to form veins. Veins: Histology (HEVs): 
      • The point of entry into lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy for most circulating lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology
      • Found primarily in the paracortex
  • Medulla:
    • Medullary cords: 
      • Closely packed masses of lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology and plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products cells 
      • Extend from the paracortex
    • Medullary sinuses:
      • Dilated spaces that are continuous with the cortical sinuses
      • Contain macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation and a meshwork of reticular cells (final filter)
      • Converge at the hilum Hilum Lungs: Anatomy, turning into the efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology lymphatic vessel

Vasculature

  • Lymph travels from afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology lymphatics → subcapsular sinus → trabecular sinuses → medullary sinuses → efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology lymphatics
  • High endothelial venules Venules The minute vessels that collect blood from the capillary plexuses and join together to form veins. Veins: Histology (HEVs):
    • Endothelium Endothelium A layer of epithelium that lines the heart, blood vessels (vascular endothelium), lymph vessels (lymphatic endothelium), and the serous cavities of the body. Arteries: Histology in HEVs has cell adhesion Adhesion The process whereby platelets adhere to something other than platelets, e.g., collagen; basement membrane; microfibrils; or other ‘foreign’ surfaces. Coagulation Studies molecules that help in the diapedesis Diapedesis Inflammation of T cells T cells Lymphocytes responsible for cell-mediated immunity. Two types have been identified – cytotoxic (t-lymphocytes, cytotoxic) and helper T-lymphocytes (t-lymphocytes, helper-inducer). They are formed when lymphocytes circulate through the thymus gland and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen. T cells: Types and Functions.
    • These venules Venules The minute vessels that collect blood from the capillary plexuses and join together to form veins. Veins: Histology orchestrate extravasation of lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology into the lymph node. 
    • After entering the lymphoid tissue, T cells T cells Lymphocytes responsible for cell-mediated immunity. Two types have been identified – cytotoxic (t-lymphocytes, cytotoxic) and helper T-lymphocytes (t-lymphocytes, helper-inducer). They are formed when lymphocytes circulate through the thymus gland and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen. T cells: Types and Functions stay in the paracortex and B cells B cells Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions migrate into nodules.

MALT

Anatomy

  • Collections of lymphoid follicles beneath mucosal linings that stand guard against antigens
  • Lymphoid follicles, similar to lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy, have germinal centers.
  • Located in the digestive, respiratory, or urogenital mucosa:
    • GI-, or gut-, associated lymphoid tissue (GALT): tonsils Tonsils Tonsillitis, Peyer’s patches Patches Vitiligo, appendix Appendix A worm-like blind tube extension from the cecum. Colon, Cecum, and Appendix: Anatomy, and lymphocyte infiltrations along the GI tract
    • Respiratory: bronchial/tracheal-associated lymphoid tissue (BALT), nose-associated lymphoid tissue (NALT)
    • Genitourinary: vulvovaginal-associated lymphoid tissue (VALT) 
  • Areas commonly invaded by pathogens due to easy entry through lumens open to the external environment
  • Contains 70% of the body’s immune cells; B lymphocytes B lymphocytes Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions are the predominant cellular population.

Main Function

Mount an immune response against pathogens encountered in the mucosa of the GI, respiratory, and genitourinary tracts.

Structure of the tonsils Tonsils Tonsillitis

  • Large and irregular collections of lymphoid tissue within the oropharyngeal cavity
  • With a pseudocapsule (made of condensed connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology
  • Variable Variable Variables represent information about something that can change. The design of the measurement scales, or of the methods for obtaining information, will determine the data gathered and the characteristics of that data. As a result, a variable can be qualitative or quantitative, and may be further classified into subgroups. Types of Variables epithelial surfaces, with deep depressions (crypts)
  • Waldeyer ring: 
    • Group of lymphoid tissues in the oropharynx Oropharynx The middle portion of the pharynx that lies posterior to the mouth, inferior to the soft palate, and superior to the base of the tongue and epiglottis. It has a digestive function as food passes from the mouth into the oropharynx before entering esophagus. Pharynx: Anatomy that provide initial protection to pathogens entering via the mouth
    • Pharyngeal tonsils Tonsils Tonsillitis:
      • Located on the posterior wall of the nasopharynx Nasopharynx The top portion of the pharynx situated posterior to the nose and superior to the soft palate. The nasopharynx is the posterior extension of the nasal cavities and has a respiratory function. Pharynx: Anatomy
      • Lined by pseudostratified ciliated columnar epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology (respiratory epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology)
      • Do not have crypts
      • Chronic inflammation Chronic Inflammation Inflammation can lead to hypertrophied tonsils Tonsils Tonsillitis (and adenoids).
    • Palatine tonsils Tonsils Tonsillitis (faucial tonsils Tonsils Tonsillitis):
      • Paired tonsils Tonsils Tonsillitis in the lateral oropharynx Oropharynx The middle portion of the pharynx that lies posterior to the mouth, inferior to the soft palate, and superior to the base of the tongue and epiglottis. It has a digestive function as food passes from the mouth into the oropharynx before entering esophagus. Pharynx: Anatomy, posteriorly on the soft palate Palate The palate is the structure that forms the roof of the mouth and floor of the nasal cavity. This structure is divided into soft and hard palates. Palate: Anatomy 
      • Covered by nonkeratinized stratified squamous epithelium Stratified squamous epithelium Surface Epithelium: Histology infiltrated with lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology
      • With smooth surface and invaginations (crypts) into tonsil parenchyma
      • Follicles/nodules are found around the crypts. 
    • Tubal tonsils Tonsils Tonsillitis:
      • Lateral wall of nasopharynx Nasopharynx The top portion of the pharynx situated posterior to the nose and superior to the soft palate. The nasopharynx is the posterior extension of the nasal cavities and has a respiratory function. Pharynx: Anatomy (around the Eustachian tube Eustachian tube A narrow passageway that connects the upper part of the throat to the tympanic cavity. Ear: Anatomy opening)
      • Forms upper lateral aspect of Waldeyer ring
      • Covered by pseudostratified ciliated columnar epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology 
      • No crypts
    • Lingual tonsils Tonsils Tonsillitis:
      • Surface of the posterior 3rd of the tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Lips and Tongue: Anatomy
      • Lined by nonkeratinized stratified squamous epithelium Stratified squamous epithelium Surface Epithelium: Histology 

Structure of Peyer’s patches Patches Vitiligo and MALT MALT Colon, Cecum, and Appendix: Anatomy in the appendix Appendix A worm-like blind tube extension from the cecum. Colon, Cecum, and Appendix: Anatomy

  • Peyer’s patches Patches Vitiligo:
    • The mucosa of the small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: Anatomy has many projecting villi.
    • Large aggregates of lymphoid nodules in the lamina propria Lamina propria Whipple’s Disease of the distal part of the small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: Anatomy ( ileum Ileum The distal and narrowest portion of the small intestine, between the jejunum and the ileocecal valve of the large intestine. Small Intestine: Anatomy
    • Covered by simple columnar epithelium Simple columnar epithelium Surface Epithelium: Histology
    • Contain specialized endothelial cells called M (or microfold) cells:
      • Have an intraepithelial pocket in the basolateral surface, which is distinctively porous to the populations of lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology and dendritic cells Dendritic cells Specialized cells of the hematopoietic system that have branch-like extensions. They are found throughout the lymphatic system, and in non-lymphoid tissues such as skin and the epithelia of the intestinal, respiratory, and reproductive tracts. They trap and process antigens, and present them to T-cells, thereby stimulating cell-mediated immunity. They are different from the non-hematopoietic follicular dendritic cells, which have a similar morphology and immune system function, but with respect to humoral immunity (antibody production). Skin: Structure and Functions.
      • Collect antigens from the intestinal epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology
      • The M cells M cells Salmonella bind BIND Hyperbilirubinemia of the Newborn the antigens, which then undergo transcytosis, which exposes the antigens to the immune cells.
      • The antigens are taken up by dendritic cells Dendritic cells Specialized cells of the hematopoietic system that have branch-like extensions. They are found throughout the lymphatic system, and in non-lymphoid tissues such as skin and the epithelia of the intestinal, respiratory, and reproductive tracts. They trap and process antigens, and present them to T-cells, thereby stimulating cell-mediated immunity. They are different from the non-hematopoietic follicular dendritic cells, which have a similar morphology and immune system function, but with respect to humoral immunity (antibody production). Skin: Structure and Functions and are presented to T helper (Th) cells. 
      • Th cells trigger Trigger The type of signal that initiates the inspiratory phase by the ventilator Invasive Mechanical Ventilation the B cells B cells Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions to differentiate into plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products cells (producing IgA IgA Represents 15-20% of the human serum immunoglobulins, mostly as the 4-chain polymer in humans or dimer in other mammals. Secretory iga is the main immunoglobulin in secretions. Immunoglobulins: Types and Functions antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions). 
      • IgA IgA Represents 15-20% of the human serum immunoglobulins, mostly as the 4-chain polymer in humans or dimer in other mammals. Secretory iga is the main immunoglobulin in secretions. Immunoglobulins: Types and Functions goes into gut lumen, where the antigen Antigen Substances that are recognized by the immune system and induce an immune reaction. Vaccination is bound and neutralized. 
  • Appendix Appendix A worm-like blind tube extension from the cecum. Colon, Cecum, and Appendix: Anatomy:
    • Close-ended, narrow, short projection Projection A defense mechanism, operating unconsciously, whereby that which is emotionally unacceptable in the self is rejected and attributed (projected) to others. Defense Mechanisms of the cecum Cecum The blind sac or outpouching area of the large intestine that is below the entrance of the small intestine. It has a worm-like extension, the vermiform appendix. Colon, Cecum, and Appendix: Anatomy
    • Filled with MALT MALT Colon, Cecum, and Appendix: Anatomy in the mucosa
    • Cases of appendicitis Appendicitis Appendicitis is the acute inflammation of the vermiform appendix and the most common abdominal surgical emergency globally. The condition has a lifetime risk of 8%. Characteristic features include periumbilical abdominal pain that migrates to the right lower quadrant, fever, anorexia, nausea, and vomiting. Appendicitis can occur with obstructions owing to lymphoid hyperplasia Hyperplasia An increase in the number of cells in a tissue or organ without tumor formation. It differs from hypertrophy, which is an increase in bulk without an increase in the number of cells. Cellular Adaptation.

Spleen

Anatomy

  • The spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy is a coffee-bean–shaped organ located in the LUQ of the abdomen, beneath the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy.
  • Anterolateral to the left kidney
  • Protected by the 9th–11th ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy
  • About 12 x 7 x 3 cm in size
  • Hilum Hilum Lungs: Anatomy (medial surface of the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy): where the splenic artery and nerves enter and the splenic vein and lymphatics leave

Main Functions

  • Site of production of antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions and activated lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology
  • Storage and breakdown of platelets Platelets Platelets are small cell fragments involved in hemostasis. Thrombopoiesis takes place primarily in the bone marrow through a series of cell differentiation and is influenced by several cytokines. Platelets are formed after fragmentation of the megakaryocyte cytoplasm. Platelets: Histology
  • Stores mononuclear phagocytes, filtering germs that are in the bloodstream
  • Site of destruction of old erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology

Structure

  • Capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides: 
    • Surrounds the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy
    • Outer dense connective tissue Dense connective tissue Connective Tissue: Histology
    • Trabeculae emerge and penetrate the splenic parenchyma or splenic pulp.
  • Red pulp:
    • Occupies most of the organ
    • Color due to high content of blood cells of all types
    • Contains mainly blood-filled splenic cords (of Billroth) and sinuses
    • Splenic cords: 
      • Contain reticular tissue with abundant T and B lymphocytes B lymphocytes Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions, macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation, other leukocytes Leukocytes White blood cells. These include granular leukocytes (basophils; eosinophils; and neutrophils) as well as non-granular leukocytes (lymphocytes and monocytes). White Myeloid Cells: Histology, and RBCs RBCs Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology 
      • Separated by the sinusoids Sinusoids Liver: Anatomy 
    • Splenic sinuses:
      • Long vascular channels Channels The Cell: Cell Membrane
      • Lined by elongated endothelial cells called stave cells (separate healthy from ineffective erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology)
      • Cells that cannot pass between stave cells are eliminated by macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation.
  • White pulp:
    • Consists mainly of lymphoid nodules and the periarteriolar lymphoid sheaths (PALS)
    • PALS: 
      • T cells T cells Lymphocytes responsible for cell-mediated immunity. Two types have been identified – cytotoxic (t-lymphocytes, cytotoxic) and helper T-lymphocytes (t-lymphocytes, helper-inducer). They are formed when lymphocytes circulate through the thymus gland and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen. T cells: Types and Functions with some macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation, dendritic cells Dendritic cells Specialized cells of the hematopoietic system that have branch-like extensions. They are found throughout the lymphatic system, and in non-lymphoid tissues such as skin and the epithelia of the intestinal, respiratory, and reproductive tracts. They trap and process antigens, and present them to T-cells, thereby stimulating cell-mediated immunity. They are different from the non-hematopoietic follicular dendritic cells, which have a similar morphology and immune system function, but with respect to humoral immunity (antibody production). Skin: Structure and Functions, and plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products cells surrounding central arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology
      • Central arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology are small branches formed from the trabecular artery.
    • Lymphoid nodules:
      • Formed when B cells B cells Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions are activated by antigen Antigen Substances that are recognized by the immune system and induce an immune reaction. Vaccination(s) in the blood.
      • Can displace the central arteriole peripherally
  • Marginal zone Marginal zone MALT Lymphoma is the region between the white and red pulp where antigen-presenting cells Antigen-presenting cells A heterogeneous group of immunocompetent cells that mediate the cellular immune response by processing and presenting antigens to the T-cells. Traditional antigen-presenting cells include macrophages; dendritic cells; langerhans cells; and B-lymphocytes. Follicular dendritic cells are not traditional antigen-presenting cells, but because they hold antigen on their cell surface in the form of immune complexes for b-cell recognition they are considered so by some authors. Adaptive Immune Response and macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation catch antigens.

Vasculature

  • Arterial supply:
    • Splenic artery ( hilum Hilum Lungs: Anatomy) → trabecular arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology → central arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology (white pulp)
    • Branches leave the white pulp → enter the red pulp (penicillar arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology) and lose the sheath of lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology (some have a sheath of antigen-presenting cells Antigen-presenting cells A heterogeneous group of immunocompetent cells that mediate the cellular immune response by processing and presenting antigens to the T-cells. Traditional antigen-presenting cells include macrophages; dendritic cells; langerhans cells; and B-lymphocytes. Follicular dendritic cells are not traditional antigen-presenting cells, but because they hold antigen on their cell surface in the form of immune complexes for b-cell recognition they are considered so by some authors. Adaptive Immune Response
    • Some capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology from the penicillar arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology dump blood into the stroma of the splenic cords:
      • Plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products and other blood elements return to the vasculature by passing through the stave cells into the sinusoids Sinusoids Liver: Anatomy
      • Functional platelets Platelets Platelets are small cell fragments involved in hemostasis. Thrombopoiesis takes place primarily in the bone marrow through a series of cell differentiation and is influenced by several cytokines. Platelets are formed after fragmentation of the megakaryocyte cytoplasm. Platelets: Histology, leukocytes Leukocytes White blood cells. These include granular leukocytes (basophils; eosinophils; and neutrophils) as well as non-granular leukocytes (lymphocytes and monocytes). White Myeloid Cells: Histology, or erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology (which are flexible) have no problem reentering the blood vessel.
      • Effete erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology cannot enter and are removed by macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation
  • Blood then leaves the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy via the splenic vein.

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Clinical Relevance

  • Hodgkin lymphoma Lymphoma A general term for various neoplastic diseases of the lymphoid tissue. Imaging of the Mediastinum ( HL HL Hodgkin lymphoma (HL) is a malignancy of B lymphocytes originating in the lymph nodes. The pathognomonic histologic finding of HL is a hodgkin/reed-sternberg (HRS) cell (giant multinucleated B cells with eosinophilic inclusions). The disease presents most commonly with lymphadenopathy, night sweats, weight loss, fever, splenomegaly and hepatomegaly. Hodgkin Lymphoma): malignancy Malignancy Hemothorax of B lymphocytes B lymphocytes Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions originating in lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy. The pathognomic histologic finding of HL HL Hodgkin lymphoma (HL) is a malignancy of B lymphocytes originating in the lymph nodes. The pathognomonic histologic finding of HL is a hodgkin/reed-sternberg (HRS) cell (giant multinucleated B cells with eosinophilic inclusions). The disease presents most commonly with lymphadenopathy, night sweats, weight loss, fever, splenomegaly and hepatomegaly. Hodgkin Lymphoma is a Hodgkin Reed-Sternberg ( HRS HRS Hepatorenal syndrome (HRS) is a potentially reversible cause of acute kidney injury that develops secondary to liver disease. The main cause of hrs is hypovolemia, often as a result of forced diuresis or drainage of ascites. This leads to renal vasoconstriction resulting in hypoperfusion of the kidneys. Hepatorenal Syndrome) cell (giant multinucleated B cell with eosinophilic inclusions). The disease presents most commonly with lymphadenopathy Lymphadenopathy Lymphadenopathy is lymph node enlargement (> 1 cm) and is benign and self-limited in most patients. Etiologies include malignancy, infection, and autoimmune disorders, as well as iatrogenic causes such as the use of certain medications. Generalized lymphadenopathy often indicates underlying systemic disease. Lymphadenopathy, night sweats Night sweats Tuberculosis, weight loss Weight loss Decrease in existing body weight. Bariatric Surgery, fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, and at times, splenomegaly Splenomegaly Splenomegaly is pathologic enlargement of the spleen that is attributable to numerous causes, including infections, hemoglobinopathies, infiltrative processes, and outflow obstruction of the portal vein. Splenomegaly and hepatomegaly. Diagnostic testing includes lymph node histologic analysis showing HRS HRS Hepatorenal syndrome (HRS) is a potentially reversible cause of acute kidney injury that develops secondary to liver disease. The main cause of hrs is hypovolemia, often as a result of forced diuresis or drainage of ascites. This leads to renal vasoconstriction resulting in hypoperfusion of the kidneys. Hepatorenal Syndrome cells. Hodgkin lymphoma Lymphoma A general term for various neoplastic diseases of the lymphoid tissue. Imaging of the Mediastinum is managed with chemotherapy Chemotherapy Osteosarcoma and radiotherapy. 
  • Tonsillitis Tonsillitis Tonsillitis is inflammation of the pharynx or pharyngeal tonsils, and therefore is also called pharyngitis. An infectious etiology in the setting of tonsillitis is referred to as infectious pharyngitis, which is caused by viruses (most common), bacteria, or fungi. Tonsillitis: inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the pharynx Pharynx The pharynx is a component of the digestive system that lies posterior to the nasal cavity, oral cavity, and larynx. The pharynx can be divided into the oropharynx, nasopharynx, and laryngopharynx. Pharyngeal muscles play an integral role in vital processes such as breathing, swallowing, and speaking. Pharynx: Anatomy or pharyngeal tonsils Tonsils Tonsillitis, thus also called pharyngitis Pharyngitis Pharyngitis is an inflammation of the back of the throat (pharynx). Pharyngitis is usually caused by an upper respiratory tract infection, which is viral in most cases. It typically results in a sore throat and fever. Other symptoms may include a runny nose, cough, headache, and hoarseness. Pharyngitis. An infectious Infectious Febrile Infant etiology in the setting of tonsillitis Tonsillitis Tonsillitis is inflammation of the pharynx or pharyngeal tonsils, and therefore is also called pharyngitis. An infectious etiology in the setting of tonsillitis is referred to as infectious pharyngitis, which is caused by viruses (most common), bacteria, or fungi. Tonsillitis is referred to as infectious Infectious Febrile Infant pharyngitis Pharyngitis Pharyngitis is an inflammation of the back of the throat (pharynx). Pharyngitis is usually caused by an upper respiratory tract infection, which is viral in most cases. It typically results in a sore throat and fever. Other symptoms may include a runny nose, cough, headache, and hoarseness. Pharyngitis, caused by viruses Viruses Minute infectious agents whose genomes are composed of DNA or RNA, but not both. They are characterized by a lack of independent metabolism and the inability to replicate outside living host cells. Virology (most common), bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology, or fungi Fungi A kingdom of eukaryotic, heterotrophic organisms that live parasitically as saprobes, including mushrooms; yeasts; smuts, molds, etc. They reproduce either sexually or asexually, and have life cycles that range from simple to complex. Filamentous fungi, commonly known as molds, refer to those that grow as multicellular colonies. Mycology. Among the bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology, group A hemolytic Streptococcus Streptococcus Streptococcus is one of the two medically important genera of gram-positive cocci, the other being Staphylococcus. Streptococci are identified as different species on blood agar on the basis of their hemolytic pattern and sensitivity to optochin and bacitracin. There are many pathogenic species of streptococci, including S. pyogenes, S. agalactiae, S. pneumoniae, and the viridans streptococci. Streptococcus is the most frequent cause. Testing for group A streptococcal (GAS) infection in the setting of tonsillitis Tonsillitis Tonsillitis is inflammation of the pharynx or pharyngeal tonsils, and therefore is also called pharyngitis. An infectious etiology in the setting of tonsillitis is referred to as infectious pharyngitis, which is caused by viruses (most common), bacteria, or fungi. Tonsillitis is essential, as this infection can lead to rheumatic fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever and glomerulonephritis in children and adolescents. Antibiotic treatment is recommended in a confirmed GAS infection for this population. 
  • Splenomegaly Splenomegaly Splenomegaly is pathologic enlargement of the spleen that is attributable to numerous causes, including infections, hemoglobinopathies, infiltrative processes, and outflow obstruction of the portal vein. Splenomegaly: enlargement of the spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen: Anatomy. Splenomegaly Splenomegaly Splenomegaly is pathologic enlargement of the spleen that is attributable to numerous causes, including infections, hemoglobinopathies, infiltrative processes, and outflow obstruction of the portal vein. Splenomegaly is a clinical finding that may be seen in disorders like lymphoma Lymphoma A general term for various neoplastic diseases of the lymphoid tissue. Imaging of the Mediastinum, infectious Infectious Febrile Infant mononucleosis Mononucleosis Infectious mononucleosis (IM), also known as “the kissing disease,” is a highly contagious viral infection caused by the Epstein-Barr virus. Its common name is derived from its main method of transmission: the spread of infected saliva via kissing. Clinical manifestations of IM include fever, tonsillar pharyngitis, and lymphadenopathy. Mononucleosis, sepsis Sepsis Systemic inflammatory response syndrome with a proven or suspected infectious etiology. When sepsis is associated with organ dysfunction distant from the site of infection, it is called severe sepsis. When sepsis is accompanied by hypotension despite adequate fluid infusion, it is called septic shock. Sepsis and Septic Shock, and rheumatic diseases. Splenic congestion (which can occur in the setting of liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy cirrhosis Cirrhosis Cirrhosis is a late stage of hepatic parenchymal necrosis and scarring (fibrosis) most commonly due to hepatitis C infection and alcoholic liver disease. Patients may present with jaundice, ascites, and hepatosplenomegaly. Cirrhosis can also cause complications such as hepatic encephalopathy, portal hypertension, portal vein thrombosis, and hepatorenal syndrome. Cirrhosis) and splenic tumors are possible causes of splenomegaly Splenomegaly Splenomegaly is pathologic enlargement of the spleen that is attributable to numerous causes, including infections, hemoglobinopathies, infiltrative processes, and outflow obstruction of the portal vein. Splenomegaly.
  • Appendicitis Appendicitis Appendicitis is the acute inflammation of the vermiform appendix and the most common abdominal surgical emergency globally. The condition has a lifetime risk of 8%. Characteristic features include periumbilical abdominal pain that migrates to the right lower quadrant, fever, anorexia, nausea, and vomiting. Appendicitis: inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the vermiform appendix Appendix A worm-like blind tube extension from the cecum. Colon, Cecum, and Appendix: Anatomy. In young patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship, lymphoid tissue underneath the mucosa undergoes a process of hyperplasia Hyperplasia An increase in the number of cells in a tissue or organ without tumor formation. It differs from hypertrophy, which is an increase in bulk without an increase in the number of cells. Cellular Adaptation (increase in the number of cells) that can cause obstruction of the lumen and initiate inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation

References

  1. Grethlein, S. (2019). Mucosa-associated lymphoid tissue lymphomas (MALTomas). Medscape. Retrieved July 9, 2021, from https://emedicine.medscape.com/article/207891-overview
  2. Mescher, A.L. (Ed.). (2021). The immune system & lymphoid organs. In: Junqueira’s Basic Histology Text and Atlas, 16th ed. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=3047&sectionid=255121636
  3. Panneerselvam, D., Budh, D.P. (2021). Peyer Patches. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK557457/
  4. Scoville, S., Caligiuri, M.A. (2021). The organization and structure of lymphoid tissues. In: Kaushansky, K., et al. (Eds.), Williams Hematology, 10th ed. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2962&sectionid=252523494

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