STUDIES ON IMMUNE HUMAN HEMOLYSIS : II. THE DONATH-LANDSTEINER REACTION AS A MODEL SYSTEM FOR STUDYING THE MECHANISM OF ACTION OF COMPLEMENT AND THE ROLE OF C'1 AND C'1 ESTERASE.

1. The inactivation, of C'1 blocks the completion of the cold phase of the Donath-Landsteiner reaction; inactivation of the other components of complement does not interfere with the cold phase of the reaction. 2. C'2, C'3, and C'4 are required for the completion of the hemolytic reaction. C'4 reacts in either the cold or warm phase, but C'2 and C'3 must react in the warm phase. 3. Partially purified C'1 or C'1 esterase can be substituted for whole serum in the cold phase, although neither reagent contains any of the other components of complement 4. Partially purified serum inhibitor of C'1 esterase blocks the effect of C'1 esterase in the cold phase, and reverses the effect of complement, C'1 or C'1 esterase when incubated with "activated" cells after the cold phase. 5. C'1 esterase activity can be eluted from "activated" erythrocytes with Na(3)EDTA. 6. The components of human complement in this human hemolytic reaction act in the order C'1, C'4, C'2, C'3. Ca(++) is required for the reaction with C'1, and Mg(++) is required for the reaction with C'2. 7. Accordingly, a functional role of C'1 esterase in a human disease state is demonstrated, and a human model is established for the study of the mechanism of action of complement.