The Stoichiometric Measurement of the Serum Inhibitor of the First Component of Complement by the Inhibition of Immune Hemolysis

Abstract

A sensitive technique for the measurement of the activity of the α-2-globulin inhibitor of the activated first component of complement has been described. The reaction is based on the ability of this inhibitor to block C′1a in a stoichiometric fashion in immune hemolysis. A hemolytic unit of C′1aINH has been defined as the amount required to inhibit 63% of one effective molecule of C′1a in the presence of a total dose of from 1.5 to 2.5 effective molecules per cell. The C′1a inhibitor is stoichiometrically depleted by its interaction with fluid phase hemolytically active C′1a. C′1aINH is equally effective in blocking C′1a in the fluid phase or on the cell. C′1aINH activity can be measured in human serum after removal of C′1 by precipitating the latter at low ionic strength, pH 7.5. Normal levels for C′1aINH activity are 37,445 ± 17,790 units/ml. Patients with hereditary angioedema were found to have a 99% reduction in C′1aINH activity.