Comparison of monocyte and complement mediated haemolysis of human A1 erythrocytes: the relationship between antibody, target and effector concentrations in the induction of lysis.

Haemolysis of sensitized A1 erythrocytes by blood monocytes or complement was measured under conditions where antibody, complement, target and effector concentrations varied relatively to each other. When sensitization was limiting, cell-mediated lysis remained constant with increasing red cell number. When antibody was in excess and complement was limiting, complement-mediated lysis remained constant with increasing red cell number. These effects could be explained if a non-homogeneous expression of A1 antigens on individual erythrocytes is assumed. Considering that the capacity of antibody to bind to the antigen is random and uniform, at high dilution of antibody only those erythrocytes with sufficiently high antigen density will be appropriately sensitized for cell-mediated lysis. At antibody excess binding sites are saturated, and therefore the number and distribution of cell bound antibodies reflect the number and distribution of antigens. The fact that under such conditions increasing target concentrations did not reduce lysis induced by a suboptimal complement concentration also argues for an uneven antigen distribution among erythrocytes.