C3 and infection: While the circulating C3a anaphylatoxin is involved in inducing inflammatory immune responses, C3b (reported (1) normal plasma concentration: ~210 ng mL-1 but levels are markedly higher on infection and under certain disease conditions although its short half-life (< 2 min) makes accurate measurements difficult) facilitates opsonophagocytosis and clearance of immune complexes through thioester-mediated opsonisation of primary amine- or hydroxyl-containing antigenic and self surfaces.