C1-INH is a critically important protein that controls activation of multiple plasma mediator pathways. This protein, a member of the serine protease inhibitor (serpin) group, originally was described as an inhibitor of C1. It binds stoichiometrically to the active sites on both C1r and C1s to form a complex C1-INH-C1r-C1s-C1-INH and thus inhibits activated C1. In addition, C1-INH has been reported to remove the intact C1qrs complex from an activating surface and to inhibit autoactivation of C1. C1-INH is a known inhibitor of kinin generating (kallikrein), fibrinolytic (plasmin), and contact activation (intrinsic) pathway of the coagulation cascade. Recently, it has been shown to be an inhibitor of the mannan-binding lectin pathway of complement activation, inhibiting mannan-binding lectin-associated serine proteases (MASPs) in that pathway.