Complement Factor H (CFH) is a serum glycoprotein with a molecular weight of 155 kD. CFH is synthesized abundantly in the liver. It is also synthesized in endothelium, macrophages, platelets, leukocytes, and several other cell types. CFH is a down-regulating protein of the alternative pathway in the complement system. CFH controls the alternative pathway in the complement system by binding C3b (C3 convertase), thereby preventing the formation of the C3 convertase complex and accelerating the dissociation of Bb from the active C3 convertase. CFH is also a cofactor for factor I (C3b inactivator) and can distinguish between activator and non-activator surfaces.

CFH deficiency has been associated with an increased tendency for several diseases, including systemic lupus erythematosus, pyogenic infection susceptibility, type II membranoproliferative glomerulonephritis, a form of chronic collagen type III glomerulopathy, and atypical familial hemolytic-uremic syndrome (HUS).