The estimation of the concentration and equilibrium constant of anti-D.

A method is described for estimating the concentration and equilibrium constant of the blood group antibody, anti-D. The principle of the method is based on the fact that the reaction between anti-D and red cells is reversible and obeys the law of mass action. D-positive red cells at five different concentrations were added to aliquots of antisera containing anti-D and the reaction allowed to come into equilibrium. The amount of anti-D bound to the red cells in each case was estimated using I-labelled anti-γ-globulin. The results of these estimates were then analysed according to two derivations of the law of mass action. Twenty-one examples of antisera containing anti-D were examined and the estimated range of values was as follows: concentration, 0.3–7.2 μg/ml; equilibrium constant, 1.4 × 10 to 1.2 × 10 1/mole; index of heterogeneity, = 0.6–0.9.