A single chain glycoprotein which inhibits C1，C1r，C1s，plasma kallikrein，Factors XIa. XIIa，and plasmin. Present in plasma at 16-33 mg per 100 ml. C1 esterase inhibitor deficiency is a rare condition resulting in facial swelling and abdominal cramping. Usually the condition is hereditary，though it may also occur when the C1EI is non-functional. The C1 esterase inhibitor protein is a normal constituent of serum which functions as a serine proteinase inhibitor of the serpin family. The C1 esterase inhibitor inhibits the complement proteases C1r and C1s，as well as the proteases kallikrein，factor XIa，XIIa and plasmin of the blood clotting system. The concentration of C1 esterase inhibitor protein is reduced to 10-30% of normal in patients with angioedema secondary to C1 esterase inhibitor deficiency (85% of patients with Hereditary Angioedema (HAE)); in 15% of patients with HAE，the concentrations of the inhibitor protein is normal but function is markedly reduced.
