Type III: immune-complex-mediated reactions. Immune complexes are formed when antigens bind to antibodies. They are usually cleared from the circulation by the phagocytic system. However, deposition of these complexes in tissues or in vascular endothelium can produce immune complex-mediated tissue injury. Two important factors leading to injury by this mechanism include increased quantity of circulating complexes and the presence of vasoactive amines, which increase vascular permeability and favor tissue deposition of immune complexes. Immune complex deposition leads to complement activation, anaphylatoxin generation, chemotaxis of polymorphonuclear leukocytes, phagocytosis, and tissue injury. Clinical examples are serum sickness, certain types of nephritis, and certain features of bacterial endocarditis.
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