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. Prepared from plasma shown to be non reactive for HbsAG, anti-HCV, anti-HBc, and negative for anti-HIV 1 & 2 by FDA approved tests. >95percent pure by SDS-PAGE.