Similar combining sites of IgG1 and IgG2a antibodies in individual mice as judged from affinity and rate constant measurements.

Within individual sera, mouse IgG1 and IgG2a subclass antibodies against the antigenic determinant oligo-D-alanine are likely to have similar combining sites. The antibodies are characterized by two methods, by their rate of binding to antigen equipped sheep red blood cells and by the hapten inhibition of antigen binding. The two characteristics are shown to be independent. The subclass antibodies of thirteen different sera differ by a factor of about 10 in their rate constant and by a factor of about 50 in affinity for hapten. In contrast, IgG1 and IgG2a within the same serum have strikingly similar rate constants (factor 1.02-1.33) as well as very similar affinities (factor 1.1-2.7). Since it is highly improbable that correlation in two independent criteria occurs by chance, the IgG1 and IgG2a antibodies are assumed to have similar combining sites.