C1-inhibitor (C1-inh, C1 esterase inhibitor) is a serine protease inhibitor (serpin) protein, the main function of which is the inhibition of the complement system to prevent spontaneous activation. 

C1-inh irreversibly binds to and inactivates C1r and C1s proteinases in the C1 complex of classical pathway of complement. MASP-1 and MASP-2 proteinases in MBL complexes of the lectin pathway are also inactivated. This way, C1-inh prevents the proteolytic cleavage of later complement components C4 and C2 by C1 and MBL. Although named after its complement inhibitory activity, C1-inh also inhibits proteinases of the fibrinolytic, clotting, and kinin pathways.