Serum complement is of potential importance to clinical medicine and to human biology in immune reactions and hypersensitivity states. However, most studies on the characteristics and mechanism of action of complement have been performed on systems largely of non-human origin (1, 2). The present studies were undertaken to study a specific immune human hemolytic disorder and to use this as a model system for the study of human complement in a reaction system involving human erythrocytes and an antibody of human origin. The Donath-Landsteiner (D-L) reaction (3), responsible for the clinical syndrome of paroxysmal cold hemoglobinuria, is a reaction which requires for hemolysis that erythrocytes be incubated with antibody and fresh serum complement first at low temperatures, usually near 0°C., and then at higher temperatures, usually 37°C. It was chosen as the model for these studies because: (a) it involves human ceils and antibody and avoids the use of reagents from multiple species (rabbit antibody, sheep red cells, guinea pig complement); (b) the D-L antibody is an active hemolysin but a weak agglutinin in contrast to the isoantibodies and cold agglutinins; and (c) the biphasic nature of the reaction permits the separation of the early and late phases of complement action.The studies in the present paper were undertaken to define the kinetics of the Donath-Landsteiner reaction and served as the basis for the observations on the mechanism of action of complement in the succeeding paper. The data indicate that many conflicting earlier observations regarding the role of complement in the reaction (3--8) may be explained on the basis of variation in (a) potency and amount of antibody used, (b) sensitivity of cells, (c) amount
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 Na3EDTA. 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.
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