The hemolytic plaque formation of cells producing antibody against heterologous albumins was tested for sensitivity to specific inhibition by free antigen. The inhibition characteristics of plaques in this system were found to be a measure for the avidity of the antibody produced by the plaque-forming cells (PFC:s). High avidity-producing PFC:s were more sensitive to inhibition than low avidity PFC:s. Immunization with a high dose of antigen induced PFC:s that produced antibody with a lower avidity as compared to PFC:s from animals immunized with a low dose. The avidity was increased with time. Determinations of avidity at the serum level were also made, and the results were in agreement with the findings at the cellular level.
The present method made it possible to demonstrate differences in avidity of antibody at the level of the single antibody-forming cell. It may also constitute a useful tool for the analysis of the cellular events leading to the production of antibodies with varying affinities during the immune response.