Mice primed to horse erythrocytes (HRBC) produced greatly enhanced 3,5-dinitro,4-hydroxyphenylacetic (NNP)-specific indirect plaque-forming cell (7S PFC) responses when given NNP.HRBC but no difference in hapten-specific direct (19S PFC) responses in comparison to non-carrier-primed mice. The effect was carrier specific and could not be produced by simultaneous challenge of rabbit erythrocyte (RRBC)-primed mice with RRBC and NNP.HRBC. When spleen cells from HRBC-primed mice were transferred to irradiated recipients, there was again an enhanced 7S response to NNP.HRBC. The primed spleen cells could be replaced by giving activated thymus cells to HRBC together with normal spleen as a source of B cells. It is concluded that T cells influence not only the amount but also the class of antibody formed by hapten-sensitive B cells
