In human, small amounts of hydrolyzed C3 (C3(H20) or C3i) are continuously formed in the fluid phase following spontaneous hydrolysis of the internal thioester bond of C3. C3i can bind to factor B which, in turn, is cleaved by factor D yielding an alternative fluid-phase C3 convertase, C3i:Bb.

Chicken factor B-like protease was shown to create a functional C3 convertase [Jensen LB & Koch C 1991]. Although an orthologue of human factor D has not been identified in the chicken genome, this chicken module describes a cleavage of factor B-like protease as an essential step of the chicken alternative complement pathway.