Computer modeling of the reaction between antigen and heterogeneous antibody in fixed antigen excess: a new method for measuring antibody affinity.

Antigen-antibody binding experiments were performed by choosing antigen excess starting conditions and then diluting both the 125I-BSA antigen and anti-BSA proportionately so that the ratio between the reactants remained constant. The fraction antigen bound was measured at each dilution. Binding data were analyzed by computer using non-linear least squares regression to determine the affinity and affinity distribution of different antisera. Early anti-BSA was found to have a unimodal distribution with a binding constant in the range of 10(6)M-1. Intermediate anti-BSA had a bimodal distribution: 1/3 high affinity (1.0 X 10(9] and 2/3 low affinity (3.4 X 10(6)M-1). Late and hyperimmune rabbit BSA antibodies had unimodal affinity distributions with binding constants varying between 1.7 and 2.9 X 10(10)M-1. Antibody affinity can be readily determined by computer analysis of binding curves obtained in constant antigen excess conditions.