The complement system is part of the innate immune system that helps defend the body against pathogens. It was discovered in 1894 and consists of proteins that flow freely in the blood and can quickly reach sites of infection. When activated, complement proteins trigger inflammation, attract immune cells, coat pathogens for destruction, and can even directly kill pathogens. There are three pathways of complement activation - the classical, lectin, and alternative pathways. Complement activation leads to the formation of the membrane attack complex that causes lysis of pathogen cell membranes.
8. Pathways of Complement Activation CLASSICAL PATHWAY LECTIN PATHWAY ALTERNATIVE PATHWAY Activation of C5 MEMBRANE ATTACK COMPLEX (MAC) antibody dependent (Ac-Ag) antibody Independent (No Ac-Ag) Activation of C3 and generation of C5 convertase
43. Biological Properties of C-activation Products Product Biological Effects Regulation as C3, but less potent (C3-INA) C4a (anaphylatoxin) opsonization; phagocytosis C4b (opsonin) C4-BP, factor I C3b (opsonin) opsonization; phagocyte activation factors H & I
44. Biological Properties of C-activation Products Product Biological Effects Regulation anaphylactic as C3, but much more potent; attracts & activates PMN causes neutrophil aggregation, stimulation of oxidative metabolism and leukotriene release C5a ( chemotactic factor ) carboxy-peptidase-C (C3-INA) C5b67 protein-S chemotaxis, attaches to other membranes