Human factor B is cleaved by factor D yielding a small fragment Ba and a surface-bound C3 convertase (C3b:Bb). Although an orthologue of human factor D is not identified in the chicken genome, published data demonstrates the presence of alternative complement pathway (ACP) activity in chicken [Otha H et al. 1983; Otha H et al. 1984; Laursen I & Koch C 1989; Mavrodis M et al. 1995; Koch C 1986]. This Reactome project annotates a hypothetical chicken factor D and describes cleavage of a factor B-like protease into an N-terminal Ba fragment of 37 kDa and a C-terminal Bb fragment of 60 kDa. These two fragments of chicken factor B-like protease were purified and characterized [Kjalke M et al. 1993].