The equilibrium constants of anti-D immunoglobulin preparations made from pools of donor plasma.

Estimates have been made of the value of the equilibrium constant and of the heterogeneity index of the reaction between anti-D and Rh positive red cells, using anti-D immunoglobulin preparations made from pools of donor plasma. The values of the equilibrium constant all fell within the range 1×10 to 10×10 1/mole, and in eighteen of the twenty-five samples the values fell within the narrow range of 2×10 to 4×10 1/mole. Estimates of the value of the heterogeneity index varied from 0.5 to 0.8. Calculations made using these values of the equilibrium constant indicate that if 200 μg of anti-D from each preparation were injected into an Rh-negative recipient with approximately 3 ml of Rr cells in the circulation, between 8 and 28 per cent (average 17 per cent) of the D antigen sites on the red cells would be bound by anti-D if equilibrium conditions were reached.