The complement system consists of a number of small proteins found in the blood, generally synthesized by the liver, and normally circulating as inactive precursors (pro-proteins). When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavages. The end-result of this activation cascade is massive amplification of the response and activation of the cell-killing membrane attack complex.
Complement component 3 (C3) plays a central role in the activation of the complement system. C3 cleavage by C3-convertase produces C3b and iC3b. C3b leads to an inflamatory response and also covalently binds to microbial cell surfaces aiding in opsonization of the microbe. C3b can be further cleaved into C3c and C3d.