Highly purified C'1 esterase of human serum is capable of inactivating isolated fourth component of human complement (ß1E-globulin). Inactivation is accompanied by changes in electrophoretic and ultracentrifugal properties of ß1E-globulin. If non-sensitized sheep erythrocytes are present during the action of C'1 esterase on ß1E-globulin, a complex is formed consisting of cells and cytolytically active fourth component (EC'4). Thus, inactivation of ß1E-globulin by C'1 esterase appears to be preceded by a state of activation enabling ß1E-molecules to combine with cell membrane receptors. Acceptor groups appear to be present also in 7S γ-globulin and in ß1E-globulin itself, since C'1 esterase can induce the formation of ß-ß and of ß1E-7S γ-globulin complexes.

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