In mammals, three pathways of complement activation - alternative, classical and lectin - merge at the key event in complement activation, cleavage of complement C3 to C3b and C3a. The proteolytic cleavage of C3 is mediated by fluid-phase C3 convertase. A single molecule of C3 convertase can enzymatically cleave hundreds of molecules of C3. The smaller fragment, anaphylatoxin C3a, initiates local inflammatory responses while the larger cleavage product C3b binds covalently to the target surface for opsonization. Opsonized surfaces are recognized by receptors on macrophages, and upon attachment are cleared by phagocytosis. In addition, C3b can bind to C3 convertases to produce C5 convertases, C4b:C2a:C3b or C3b:C3b:Bb, which cleave C5 into C5a and C5b. C5b is an active fragment which initiates the membrane attack complex (MAC) formation (Janeway CA et al 2001).

Chicken complement factor C3 has been isolated, mapped and characterized [Laursen I and Koch C 1989; Mavrodis M et al 1995]. Chicken C3 consists of 1652 amino acids and shares 54% identity with its human and mouse counterparts. Like its mammalian orthologs chicken C3 has a two-chain structure with an alpha chain (11kDa) and a beta-chain (70kDa). Upon complement activation chicken C3 is cleaved into fragments that resemble mammalian C3a and C3b [Laursen I and Koch C 1989].