Fig. 1

Schematic representation of activation pathways of the complement system. Activation of the classical, mannose-binding protein (MBP) and alternative pathways leads to formation of the C3 convertases (C4b2a for the classical and MBP pathways and C3bBb for the alternative pathway). C3 convertases cleave C3 into C3a and C3b and complement host defense functions are mediated by the binding of C3a to the C3a receptor (C3aR), and C3b and fragments derived from additional cleavages of C3b bind to the complement receptors 1 through 4 (CR1–4). C5 convertases are formed by the binding of C3b molecules to C3 convertases (derived from either pathway) and these convertases cleave C5 into C5a and C5b. C5a binds to the C5aR and mediates many of the inflammatory functions of the complement system (see text), while C5b starts the formation of the membrane attack complex (MAC), a multimolecular complex that can insert itself into lipid bilayers, leading to lysis. Listed next to the pathways are many of the functions mediated by complement on activation.