Complement Factor I
Complement factor I (FI) is a plasma serine protease that functions strictly as a negative regulator: it cleaves C3b to iC3b and further to C3dg, and C4b to C4c and C4d, but only when a cofactor such as factor H, C4BP, CR1, or membrane cofactor protein is present. This dependence makes factor I the enzymatic arm through which surface-bound regulators protect host tissue while allowing opsonin fragments to accumulate on unprotected microbes. The protein consists of heavy and light chains joined by a disulfide bond, with the catalytic activity residing in the light chain. Complete factor I deficiency depletes C3 and predisposes to severe pyogenic infections.
