Changes and persistence of idiotypic specificities of specifically purified rabbit anti-p-azobenzoate antibodies were studied by quantitative methods. In each rabbit idiotypes identified 2 months after the start of immunization were still present in comparable concentrations 2 months later. After month 4, they were replaced by new and unrelated specificities; the changes were abrupt in two rabbits and gradual in the third, and were associated with an increase in the average affinity for specific hapten. In two surviving rabbits the new sets of specificities persisted in part for at least 1 yr. Quantitative changes occurred during this period, and the antibody preparation used as immunogen reacted most effectively with the homologous anti-D serum. The antibody population present at month 17 (D17) in one rabbit was deficient in idiotypic specificities present in D8 and lacked specificities present in D2, indicating the presence in D17 of a third group of specificities. The percentage of the antibody population from each rabbit reactive with homologous anti-idiotypic serum was greater at month 8 than at month 2, suggesting a decrease in heterogeneity.

Since the donor rabbits were challenged repeatedly with antigen, it appears that, after month 8, a portion of the antigen was utilized to stimulate existing cell lines and a portion to initiate new clones.

Precipitation of anti-p-azobenzoate antibodies removed idiotypic specificities, indicating that they were not present on the anti-bovine γ-globulin antibodies in the same sera.

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