The kinetics of the generation of primed IgM and IgG antibody-forming cell precursors, and of helper T-cell populations, were analyzed in mice whose primary responses to high and low doses of SRBC were arrested at intervals by the immunosuppressive agents cyclophosphamide monohydrate and specific antibody. The extent to which immunological memory was established in these animals before blockade of the primary response was assessed by the hemolytic plaque assay following challenge 12 wk after priming. The presence of IgG B-memory cells and T-memory cells in suppressed mice was further investigated by the transfer into these animals of syngeneic SRBC-stimulated thymocytes or anti-θ-treated spleen cells.

It was found that the progenitors of secondary IgM-synthesizing cells were primed almost immediately after injection of antigen, and that early blockade of the primary response resulted in a raised IgM response after challenge. On the other hand, priming for a secondary IgG response took at least 4 days, and was dose-dependent, although helper T populations for a secondary IgG response appeared 3 days after antigen injection. It appeared that both IgM and IgG memory cells may be considered as Y cells in terms of the X-Y-Z scheme of lymphocyte activation, but that the two populations are generated at different times after exposure to antigen. The size of either Y-cell population at any given time is dependent upon the amount of antigen available to provoke differentiation to antibody-forming Z cells, and the IgM Y-cell population in particular is likely to be depleted during the course of a normal 1° response. When IgM Y cells were maintained for long periods as a result of immunosuppression, their secondary antibody response was independent of the primed T cells necessary for a secondary IgG response.

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