Complement factor I (CFI) is a serine protease that inhibits all complement pathways by degrading surface-bound C3b and C4b into inactive fragments. Factor I binds to CFH:C3b complex and cleaves C3b, producing iC3b which remains bound to the membrane. iC3b is proteolytically inactive and cannot contribute to the complement cascade process, though it does contribute to opsonization [Ross JD et al 1982; Masaki T et al 1992; Fukui A et al 2002; Nilsson SC et al 2010].
Predicted chicken CFI shows 51% amino acid sequence identity with its human counterpart. The chicken events of complement factor I activity have not been verified experimentally but are inferred from the orthologous human events.