immunology allergic disorders
Complement System
Biologic activities of complement
Complement components have other immune functions that are mediated by complement receptors (CRs) on various cells. Several CRs use molecules that have been assigned a CD number. CR1 (CD35) promotes phagocytosis and helps clear immune complexes. CR2 (CD21) regulates antibody production by B cells and is the Epstein-Barr virus receptor. CR3 (CD11b/CD18), CR4 (CD11c/CD18), and C1q receptors play a role in phagocytosis. C3a, C5a, and C4a (weakly) have anaphylatoxin activity: They cause mast cell degranulation, leading to increased vascular permeability and smooth muscle contraction. C3b acts as an opsonin by coating microorganisms and thereby enhancing their phagocytosis. C3d enhances antibody production by B cells. C5a is a neutrophil chemoattractant; it regulates neutrophil and monocyte activities and may cause augmented adherence of cells, degranulation and release of intracellular enzymes from granulocytes, production of toxic oxygen metabolites, and initiation of other cellular metabolic events. 