Type III hypersensitivity

Type III hypersensitivity
Immune complex
Classification and external resources
MeSH	D007105
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Type III hypersensitivity occurs when there is accumulation of immune complexes (antigen-antibody complexes) that have not been adequately cleared by innate immune cells, giving rise to an inflammatory response and attraction of leukocytes. Such reactions progressing to the point of disease produce immune complex diseases.

Type III hypersensitivity occurs when there is an excess of antigen, leading to small immune complexes being formed that fix complement and are not cleared from the circulation. It involves soluble antigens that are not bound to cell surfaces (as opposed to those in type II hypersensitivity). When these antigens bind antibodies, immune complexes of different sizes form. Large complexes can be cleared by macrophages but macrophages have difficulty in the disposal of small immune complexes. These immune complexes insert themselves into small blood vessels, joints, and glomeruli, causing symptoms. Unlike the free variant, a small immune complex bound to sites of deposition (like blood vessel walls) are far more capable of interacting with complement; these medium-sized complexes, formed in the slight excess of antigen, are viewed as being highly pathogenic.

Such depositions in tissues often induce an inflammatory response, and can cause damage wherever they precipitate. The cause of damage is as a result of the action of cleaved complement anaphylotoxins C3a and C5a, which, respectively, mediate the induction of granule release from mast cells (from which histamine can cause urticaria), and recruitment of inflammatory cells into the tissue (mainly those with lysosomal action, leading to tissue damage through frustrated phagocytosis by PMNs and macrophages).

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