System of hepatically synthesized plasma proteins that play a role in innate immunity and inflammation. Membrane attack complex (MAC) defends against gram ⊝ bacteria.

Activation pathways:- Classic – IgG or IgM mediated.- Alternative – microbe surface molecules.- Lectin – mannose or other surgars on microbe surface.

Functions:- C3b – opsonization- C3a, C4a, C5a – anaphylaxis- C5a – neutrophil chemotaxis- C5b-9 – cytolysis by MAC

Opsonins – C3b and IgG are the two 1° opsonins in bacterial defense; enhance phagocytosis. C3b also helps clear immune complexes.Inhibitors – decay-accelerating factor (DAF, aka CD55) and C1 esterase inhibitor help prevent complement activation of self cells (eg, RBCs).