The complement system is a set of 14 serum proteins that are synthesized by cells and circulate in plasma as inactive forms. When activated, they are converted into active enzymes that lyse cells. The classical pathway begins with activation of C1, followed by C4, C2, and C3, then C5 to C9, terminating in cytolysis. Complement plays a role in destroying antigens through antibody reactions and contributes to processes like chemotaxis and opsonization. It can be activated through both serologic and nonserologic reactions.

1. JULIETO C. TUMULAK, RMT