Human complement factor H exists in 2 forms. The most common form, of 150kD, and the less common form of 43kD. Factor H is secreted by the liver into the blood serum. It is important in regulating the complement pathway, preventing unnecessary inflammation which can damage the host tissue. Complement factor H functions as a cofactor in the inactivation of C3b by factor I. It makes C3b susceptible to cleavage by factor I, resulting in iC3b. Factor H also inhibits the formation of the C3bBb complex (C3 convertase) and increases the rate of dissociation of both C3 convertase and the (C3b)NBB complex (C5 convertase). This prevents these components of the classical and of alternative complement pathways from forming a positive feedback loop. Mutations in factor H are associated with hemolytic-uremic syndrome, age-related macular degeneration, membranoproliferative glomerulonephritis (MPGN) type II and chronic hypocomplementemic nephropathy.