The lymphoid organs, including thymus, spleen (lien), lymph nodes and tonsils are exciting focal points and an important part of medical studies. The lymphatic system is involved in numerous processes and exerts its effects throughout the entire body. It serves a purpose in immune defense and is very complex. Understanding this system is essential for the diagnosis of many pathological processes. This article deals with the structure, modes of action and an initial overview of the diseases of the lymphatic system.
Anatomy of the Lymphatic System

Image: “Anatomy of the Lymphatic System” by philschatz. License: CC BY 4.0

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The liquid from the tissue, filtered through the capillaries, is transported through the lymphatic system. Lymph is comprised of tissue fluid and contains proteins and excess cell fluid, metabolites and foreign matter. It also contains many nutrients and transports, inter alia, the following:

  • Sodium
  • Calcium
  • Creatinine
  • Glucose
  • Lipase
  • Phosphate
  • Catalase
  • Urea

If the flow of lymph comes to a halt, materials such as fibrinogen ensure that the lymph coagulates, encapsulating the lymphocytes. Furthermore, it contains the lymphocytes for defense. About two liters of lymph are produced daily.

Strictly speaking, the lymph is the link between tissue fluid (intercellular fluid) and blood plasma and has a slightly turbid and milky appearance. Lymph is divided into lymph serum (supernatant lymph) und chylus (fatty lymph from the digestive tract).

Lymph originates from extracapillary fluid that is no longer absorbed by the bloodstream. It is first collected in the lymph capillaries, which form to build larger vessels. These vessels then lead to the lymph nodes. The lymph nodes have the task of collecting the lymph, as well as to filter it. Here, efferent lymphatic vessels (vasa efferentia) ensure that the lymph is transported onwards.

Lymphatic System

The lymphatic system is a system comprised of fine vessels. It consists of the lymphatic vessels and lymphoid organs. The purpose of the lymphoid organs is to form lymphocytes, which are essential for the immune system of the body.

A first distinction can be made between primary and secondary lymphoid organs. The primary lymphoid organs include the thymus and the bone marrow. They are characterized by specialized tissue, where the formation and maturation of B and T lymphocytes takes place. Before birth, the liver is also a primary lymphatic organ. The lymphocytes that are formed here migrate into the secondary lymphoid organs.

Among the secondary lymphoid organs are the lymph follicles of the mucous membranes, as well as the Peyer’s patches (located in the small intestine), the appendix vermiformis, the tonsils, the lymph nodes and the spleen (specifically the white pulp).

The lymph vessels on the other hand, derive the lymph from the tissues. The task of the lymphatic system is the exchange of substances. The lymphatic system is not self-contained. One example of a closed system is the bloodstream. The lymphatic vessels are located in large parts alongside arteries and veins, which facilitates the exchange. The wall of the lymph vessels is thin and porous, facilitating the uptake of substances from the environment, which are then transported further.

Lymph nodes

Every human being has about 600 lymph nodes, many of which are located in the neck, the armpits, the groin and near the gastrointestinal tract. Lymph nodes are very small, with a size of less than 1 mm. However, they are increasing in circumference when it comes to diseases.

In such cases, the lymph nodes may be up to 3 cm large. Primary follicles, in turn, are spherical cell clusters made up of lymphocytes. This is where the contact between lymphocytes and antigens (exogenous structures) takes place. Once the ability to fight is developed, they are called secondary follicle which have germinal centers. Here, the lymphocytes mature and are thus turned into immune cells.

Structure and Histology of a Lymph Node

Image: “Structure and Histology of a Lymph Node” by philschatz. License: CC BY 4.0

Filtering the Lymph

In the inguinal lymph nodes, the lymph from legs, abdomen and buttocks is filtered. In the axillary lymph nodes (axillary lymph nodes), the filtering of the lymph from the anterior and posterior abdominal wall and the arms takes place. The lymph nodes in the neck (cervical lymph nodes) are responsible for filtering the lymph from the head.

From said lymph nodes, the lymph is conducted into the inside of the body. For this reason, a change of the lymph nodes is the first indication for a physician as to which organ is ill. Each organ has its own nearby lymph nodes, which changes during pathological processes. However, diseases might also wander through the lymph and infect additional lymph nodes.

The Spleen (Lien) – a Lymphatic Organ

The spleen is the largest lymphatic organ and weighs approximately 150-200 grams. It is about 12 cm long and 7 cm wide, with a thickness of more than 4 cm. It is located below the diaphragm in the left upper abdomen, inside a firm capsule of connective tissue. The spleen itself is traversed by connective tissue and is composed of reticular fibers and cells.

Its task is the formation of white blood cells (leukocytes) and their storage. In addition, it filters aging red blood cells (erythrocytes) from the blood. The spleen also recognizes bacteria and viruses and is thus important for immune defense.

However, for adults it is not essential for survival. Still, people whose spleen was removed are significantly more susceptible to infections. The spleen has two functions and is subdivided accordingly into the red and white pulp. The organ has a similar cleansing function as the lymph nodes, however, it is “in charge” of the entire bloodstream.

Red and white pulp

Red and white pulp

Image: Red and white pulp.” by DooFi. Licence: Public Domain

The Malpighian corpuscles, also called spleen nodules, are located inside the spleen, forming the white pulp. The heavily perfused gaps represent the red pulp. The white pulp produces lymphocytes; whereas in the red pulp, old erythrocytes are broken down.

Additionally, the red pulp is the storage location for platelets and also stores leukocytes, which are then delivered as needed. Furthermore, pre-damaged blood cells are broken down or substances that can cause diseases are separated.


The Thymus

Location, Structure, and Histology of the Thymus

Image: “Location, Structure, and Histology of the Thymus” by philschatz. License: CC BY 4.0

The thymus is located behind the breastbone and is a lymphatic organ without an excretory duct. The organ consists of two asymmetrical lobes that are connected to each other. Connective tissue structures further divide these lobes into small sections. Here, T-lymphocytes mature in preparation for their role in immune defense.

In neonates, the thymus is still quite large (about 30 grams in weight), but then regresses during puberty until it weighs only about 18 grams in adulthood. It is normal for the thymus to become fatty, and glandular tissue disappears. These fatty deposits serve no function.

The functions of the thymus are of a more general nature and involve the formation and differentiation of T-lymphocytes, which are especially important for immune defense during childhood. Under the microscope, the tissues of the thymus show an outer layer with a high cell volume. Located on the inside is marrow containing Hassall’s corpuscles,whose function is not yet clear.


The tonsils are lymphoid organs as well. They are located inside the oral cavity and the throat. They include the palatine tonsils, the adenoids (pharyngeal tonsils),the tubal tonsils and lingual tonsils. Together they form Waldeyer’s throat ring, which is well supplied with blood during colds and visible when the mouth is opened.

All tonsils contain lymph follicles. Inside of them, the B-lymphocytes multiply as part of the specific immune response. In case of close contact with pathogens, an even stronger forming of lymphocytes is triggered. This causes the tonsils to swell, which is perceived as an extremely painful process.

Locations and Histology of the Tonsils

Image: “Locations and Histology of the Tonsils” by philschatz. License: CC BY 4.0

The tonsils are an important part of the immune system. They are designed to prevent pathogens from entering. The tonsils are significantly larger in children; however, starting from puberty they become much smaller. In adults, the tonsils hardly have any important functions left, because they mainly serve in the development of the immune system. During examinations, swollen tonsils are clearly visible when the mouth is opened. Tonsils function as an early warning system and control the various access points through which germs can penetrate.

Structure of the tonsils

The tonsils are part of the mucosa-associated lymphoid tissue (MALT). On closer inspection, crypts can be seen (indentations). They exist to increase the surface area. In adults, the total surface area of the tonsils amounts to about 300 cm².

The surface consists of stratified squamous epithelium. In these indentations with their spongy surface, pathogens can “get stuck” easily. Pathogens that are already known to the immune system can be detected at this point. However, unknown pathogens can penetrate further. The tonsils are actually a collection of lymph nodes, which differentiate themselves like connective tissue. Structurally, however, all the tonsils are the same. they are also all located below the mucosa.

Mucosa-associated Lymphoid Tissue (MALT) Nodule

Image: “Mucosa-associated Lymphoid Tissue (MALT) Nodule” by philschatz. License: CC BY 4.0

Diseases of the Lymphatic System

If a disease of the lymphatic tissue is not detected and treated early enough, it can spread. The best example for this is tonsillitis, which can cause serious health issues, such as endocarditis, if left untreated.

Hodgkin’s Lymphoma

Hodgkin’s lymphoma is a very aggressive disease and targets the lymphatic system. Symptoms include swollen lymph nodes that are not painful. Other signs include fatigue, weight loss and fever. The earlier the disease is diagnosed, the greater the chances of recovery.

Non-Hodgkin Lymphoma

This disease includes an enlargement of the lymph nodes, also called lymphoma. A lymphoma can be benign, as well as malignant. Only lymphomas that are malignant, but are not a Hodgkin’s lymphoma, are called non-Hodgkin lymphoma.

Diseases of the Spleen

The splenomegaly is a possible disease of the spleen. This is an enlargement of the spleen. Nevertheless, a splenomegaly is not a separate disease but may be the result of various triggers. These may be infectious diseases like infectious mononucleosis. But even sepsis or a rheumatic disease can be its cause.

Congestion is also a theoretic possibility. This involves blood backing up in the spleen, which can be the case in the wake of liver cirrhosis. Ultimately, a tumor disease is possible as well, which leads to an increase in the size of the spleen.

Diseases of the tonsils


Image: “Tonsillitis” by Michaelbladon. License: Public Domain

Tonsillitis is an inflammation of the tonsils; the palatine tonsils are mostly affected. In medical terms, it is referred to as angina tonsillaris. This is an acute bacterial infection. Streptococci often cause it, but other pathogens are possible as well. These include staphylococci or pneumococci.

People whose immune system is weakened are particularly vulnerable. They experience cases of repeated tonsillitis. Also in children, the disease is rather frequent, which can result in enlarged adenoids. The result is displaced breathing, which may require surgical removal of the tonsils. Gargling with soothing solutions may suffice as analgesic treatment in cases of minor symptoms. Even home remedies or herbal medicines often suffice. With severe symptoms however, administration of antibiotics is required.

Popular Exam Questions on Lymphoid Organs

Solutions can be found below the references.

1. The lymph…

  1. …can coagulate.
  2. …does not contain proteins.
  3. … is made up of only tissue fluid.
  4. …primarily contains sodium.
  5. … is always milky.

2. The red pulp…

  1. …stores lymphocytes.
  2. …produces lymphocytes.
  3. …breaks down leukocytes.
  4. …emits platelets.
  5. …is located outside of the spleen.

3. The thymus…

  1. … is vital during youth and in old age.
  2. … doubles its weight through fatty deposits.
  3. … has to develop in newborns.
  4. … contains glandular tissue for a lifetime.
  5. … is important in children for the development of the immune system.

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