After a single intravenous injection of rabbits with aggregated HuIgG, IgM- and IgG-plaque-forming cells (PFC) in both the spleens and peripheral blood of rabbits peaked 5, 13, and 21 days after injection, while almost no PFC could be detected on days 8 and 16. The available data suggest that the secondary peaks of PFC (days 13 and 21) resulted from stimulation of memory cells by persisting antigen that was localized in the germinal centers in the spleen. No such persistence of antigen occurred in the lymph nodes, and these lymphoid tissues did not exhibit secondary peaks of PFC.

The identical kinetic patterns for IgM- and IgG-PFC indicate that the major portion of IgG-PFC did not result from IgM-secreting cells switching to IgG synthesis and secretion.

The present data suggest that the antibody produced and present at the site of interaction between committed cells and antigen is responsible for the regulation of antibody synthesis to persisting antigens. Possible cellular events involved in both the regulation and an apparent synchronous appearance of antibody producing cells in the spleens of rabbits were presented.

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