In mammals, C3b binds covalently to a specific site on the C4b subunit of the C3 convertase resulting in the formation of the complex C4b:C2a:C3b, which functions as a C5 convertase. Here we assume that avian C5 convertase is formed in a similar way. A homologue for C2 was not found in chickens and factor B seemed to participate in both classical and alternative pathways of complement activation (Barta O & Hubbert NL 1978; Kjalke M. et al. 1993). It was assumed that the role of C2 may be fulfilled by the chicken factor B-like protease, an evolutionary remnant of a common C2/factor B ancestor (Kjalke M. et al. 1993).