C1 INH (C1 esterase inhibitor) is a protease inhibitor belonging to the serpin superfamily. Its main function is the inhibition of the complement system to prevent spontaneous activation. The protein is an acute-phase protein that circulates in blood at levels of around 0.25 g/L. The levels rise 2-fold during inflammation. C1 INH irreversibly binds to and inactivates C1r and C1s proteases in the C1 complex of classical pathway of complement. MASP-1 and MASP-2 proteases in MBL complexes of the lectin pathway are also inactivated. This way, it prevents the proteolytic cleavage of later complement components C4 and C2 by C1 and MBL. Although named after its complement inhibitory activity, the protein also inhibits proteases of the fibrinolytic, clotting, and kinin pathways. The protein is the most important physiological inhibitor of plasma kallikrein, fXIa, and fXIIa.