In mammals, the classical pathway is initiated by binding of antibody molecules (IgM or IgG) to an antigen, followed by binding of complement protein C1q. Two molecules each of C1r and C1s bind to C1q to form an active C1 complex. The activated C1 complex cleaves C4 to generate C4a and C4b. C4b fragment binds the nearby target cell surface. The activated C1 complex then cleaves and activates C2 which has bound to C4b, yielding a C4b:C2a complex (C3 convertase). C3 convertase, in turn, cleaves the third component of complement C3 to form C3a and C3b. A single C3 convertase can generate hundreds molecules of C3b, which can directly bind to the target surface. Opsonization with C3b fragments mark the target cell for phagocytosis. C3b can also bind to the C3 convertase yielding C5 convertase (C4b:C2a:C3b), which cleaves C5 into C5a and C5b. C5b is an active fragment which initiates a membrane attack complex (MAC) formation (Janeway CA et al. 2001).