The increase in splenic plaque-forming cell (PFC) responsiveness which occurs in mice following the injection of a primary dose of 108 sheep red blood cells was measured using the spleen cell transfer system. As for grafts of normal spleen cells, the peak direct (19S) and indirect (7S) responses obtained with grafts of primed spleen cells were found to be related to the number of cells transferred in a near-parabolic fashion. Splenic direct PFC responsiveness as measured by the regression defining the corresponding graft-response relationship was increased about five times 4 weeks after priming; focus-forming capacity was also increased by a similar factor. At the same time indirect PFC responsiveness was increased almost twenty times. These results are discussed in terms of antigen-induced changes in the populations of PFC precursors and antigen-reactive auxiliary cells in the spleen.
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