Native, human C1-esterase. Complent C1-esterase inhibitor purified from human plasma. Single chain glycoprotein present in normal human serum at a concentration of 180 μg/ml. Involved in regulation of complement, coagulation, fibrinolytic and contact (kinin-forming) systems of circulating blood plasma. It is the only known inhibitor of activated C1r and C1s complement enzymes. Persons with either hereditary or acquired deficiencies of C1 esterase inhibitor are susceptible to recurrent attacks of angioedema.