Complement supports host defense, but its activation on host tissues or inadequate regulation can contribute to disease.
Activation and effector mechanisms
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Different complement activation pathways converge on cleavage of C3.
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Complement functions are mediated by breakdown products of C3 and other complement proteins, together with the membrane attack complex (MAC).
Disease associations and treatment
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Complement activation by antibodies or antigen–antibody complexes deposited on host cells and tissues is an important mechanism of cell and tissue injury.
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Inherited deficiencies of complement proteins increase susceptibility to infections, while deficiencies of regulatory proteins cause various disorders.
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Diseases involving excessive complement activity, including hemolytic uremic syndrome and paroxysmal nocturnal hemoglobinuria, often carry an increased risk of thrombosis through uncertain mechanisms.
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Antibodies that block complement activation have been developed to treat several complement-associated disorders.