Complement activation occurs by two different sequences, the classic and alternative pathways.
Components C3 and C5 are involved in the generation of anaphylatoxin and the promotion of leukocyte chemotaxis, the result of those two activities being the inflammatory response. C1 and C4 are involved in the neutralization of viruses. The alternative pathway can be activated by IgA and IgG immune complexes and also by nonimmunologic materials including bacterial endotoxins and cell walls. Complement activation also results in the formation of many biologically active complement fragments that act as anaphylatoxins, opsonins, or chemotactic factors. All rights reserved.