Factor H

Factor H is the dominant fluid-phase regulator of the alternative pathway: a 150-kDa plasma protein composed of twenty short consensus repeat (SCR) domains arranged in a flexible chain. Through its N-terminal SCR1-4 region it serves as a cofactor for factor I-mediated cleavage of C3b, accelerates decay of the C3bBb convertase, and distinguishes self from non-self by binding sialic acid and glycosaminoglycans on host surfaces with its C-terminal domains. Mutations or autoantibodies against factor H cause atypical hemolytic uremic syndrome and C3 glomerulopathy, and common variants modify age-related macular degeneration risk. Recombinant factor H supports these regulatory studies.