Complement and cold agglutinins. II. Interactions of the components of complement and antibody within the haemolytic complex.

Immune haemolysis by cold agglutinins allows the study of the reactions of the components of complement after elution of antibody from the haemolytic complex. Data are presented which indicate: (1) Elution of cold agglutinin from EAC' is accompanied by elution of C' esterase and apparent loss of activity of C' from the complex. The remaining EC' does not haemolyse in whole complement, but will lyse in either whole complement or R providing antibody is replaced. Lysis of EC' in R was performed with anti-A and the Donath–Landsteiner antibody as well as with cold agglutinins. It was concluded that C' is bound firmly to a cell receptor site during complement reactions, that these sites are shared by at least three antigenic systems, and that C' remains functionally active on the cell. (2) The presence of the components of complement, especially C', exert a weak binding or stabilizing effect upon cold agglutinin within the haemolytic complex. (3) The presence of C' and C' on cells is associated with decreased number of antigenic sites for cold agglutinins and a decrease in sensitivity of the cells in immune haemolysis. (4) Cold agglutinin elutes from EAC' unchanged physicochemically or in haemolytic activity. Complexing of complement with cold agglutinin during immune haemolysis did not occur.