Inflammation is the physiologic body reaction in response to injury. The body tries to protect itself from harmful pathogens and physical or chemical irritants. Immune cells, fibroblasts, intra- and extra-cellular mediators are all involved in protecting the body, clearing the inflammatory products and help with the healing damaged parts. Sometimes the whole pathologic disorder is secondary to the body's reaction and inflammation as in cases of hypersensitivity, granulomas and autoimmune diseases. The following article describes various forms of inflammation as well as the body's reaction to them.
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acute appendicitis

Image: “Acute appendicitis.” by Ed Uthman from Houston, TX, USA. License: CC BY 2.0

Causes of Inflammation

Infections by bacteria, viruses, fungi, parasites or due to physical injury, radiation, burns, trauma or chemical injury, e.g. toxins and alcohol can all cause inflammation.

Clinical signs of inflammation include redness, hotness, swelling, pain and loss of bodily functions. Redness and hotness are due to increased blood flow to the inflamed site while swelling is due to exudate of plasma and lymph. Pain is mediated by cytokines like bradykinins and prostaglandins while the functioning is determined by the affected organ.

Pathophysiology and Stages of Inflammation

Inflammation can be acute or chronic. Acute inflammation is the immediate innate body response to harmful agents which are usually bacterial pathogens and physical tissue injuries. The involved cells which are part of the cellular complexity of the inflamed tissue e.g. macrophages, histiocytes and Kupffer cells, recognize pathogens and damaged cellular debris through pathogen-associated molecular patterns and damage-associated molecular patterns, PAMs and DAMs. These are surface receptors that recognize pathogenic antigens and cellular debris.

Activation of these surface receptors leads to release of histamine, nitric oxide and prostaglandins from tissue macrophages, mast cells and vascular endothelial cells. These mediators are responsible for the dilatation of blood vessels at the inflamed area and increase capillary permeability known as vascular phase.

Fluid shift into the inflamed tissue space will establish an inflammatory response which results in redness and hotness with exudation of fluids containing proteins e.g. fibrin, antibodies and complement proteins. These proteins function to fight the invading pathogen and limit the infection process.

Exudation of plasma from the blood will lead to stasis with subsequent extravasation of inflammatory cells from stagnant blood to the injured tissues which is known as cellular phase.

Vascular phase

Tissue macrophages and mast cells release inflammatory mediators, which lead to vasodilatation and increase tissue permeability. These mediators are also responsible for recruiting more inflammatory cells from the blood to the site of injury.

Inflammatory cells function to phagocytize foreign materials, promote the immune response and secrete other mediators. Inflammatory cells in acute inflammation are mainly granulocytes while in chronic inflammation they are lymphocytes and monocytes.

Extravasation of cells from the blood to the tissue space starts with margination of the cell towards the endothelial surface of the blood vessels due to interleukins and TNF-α. These mediators enhance the expression of ICAM-1, P-selectin and E-selectin on the endothelial cell surface to induce their adhesion with blood leukocytes before their migration.

Cellular phase

Chemokines-activated leukocytes find their way to the inflamed tissue through transmigration across the endothelium and basement membrane via acquiring foot processes to move between endothelial cells; a process called diapedsis.

Chemotactic factors e.g. C3a and C5 then guide leukocytes towards the site of tissue injury within the intercellular space via protein adhesions and binding to tissue integrins.

Neutrophils that migrate to the inflammation site act as phagocytes for the foreign organisms and cellular debris. The cells destroy the engulfed microorganisms via lysosomal enzymes and reactive oxygen species that can be introduced later as antigen presenting cells.

Phagocytosis is the process of engulfing foreign materials by phagocytes. These foreign materials include pathogens, dead cells, foreign debris and abnormal cells. Phagocytes can recognize the foreign material via surface receptors including PRRs that bind to PAMPs,  opsonin receptors that bind to bacteria coated with complement and antibodies opsonization proteins and scavenger receptors.

Opsonization occurs after complement protein C3b with the foreign antigen binds to CR1 complement receptor on the surface of phagocytes to facilitate its immune destruction.

Types of Inflammation

  1. Serous inflammation occurs with mild irritation of the epithelial or mesothelial surfaces with resultant homogenous watery exudate from the plasma or lymph. It is more common on the skin and mesothelial membranes.
  2. Mucinous or catarrhal inflammation occurs with cuboidal cells where the exudate is mainly mucin. It is more common in the respiratory tract.
  3. Purulent inflammation occurs when the inflammatory process is secondary to violent organisms e.g. staphylococci that result in large amounts of pus composed of neutrophils and dead cells. Pus isolation with fibrosis and histiocytes is known as abscess.
  4. Fibrinous inflammation occurs in fibrin-rich exudate which will organize to form fibrous mesh. It occurs mainly in viral inflammation and chemical or physical irritation. Fibrinous inflammation will lead to a permanent dysfunction in the affected tissue as fibrosis lack parenchymal cells e.g. scars on the skin surface, liver fibrosis in viral and alcoholic hepatitis. Fibrosis can also occur on the surface of tissues leading to pseudo-membrane formation which is a devitalized membrane of fibrous tissue as in pseudo membranous colitis.
  5. Hemorrhagic inflammation occurs with exudate full of erythrocytes or blood. It is common with viral infection and physical irritation. The hemorrhagic exudate is rich in cellular components and fibrin, which form into a clot limiting organ functions.
  6. Ulcerative inflammation occurs with loss of necrotic epithelial integrity leading to ulcer formation on epithelial surfaces.
  7. Granulomatous inflammation occurs with persistent inflammation leading to formation of walls of histiocytes, known as granulomas, surrounding the area of inflammatory focus. It is common in chronic inflammatory conditions e.g. tuberculosis and sarcoidosis.

Fate of Acute Inflammation

Interestingly, these mediators are short-acting and they are active only as long as there is a pathogenic stimulation. After resolution of the offending stimulus, fibrin mesh act to guide wound healing and resolution of the inflammatory products by macrophages and lymphatic drainage to the regional lymph nodes.

Persistence of the offending agent will lead to fibrosis to control the infection, abscess formation or chronic inflammation lasting for months or years with delayed healing. Chronic inflammation is characterized by replacement of neutrophils with macrophages and fibroblasts releasing IFN-γ and reactive oxygen species characterized by more fibrosis and more tissue destruction.

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