Abstract
We have found that sub-immunogenic doses of sheep red blood cells (SRBC) in vivo increase the plaque-forming cell (PFC) responses to trinitrophenyl and to SRBC when spleen cells are challenged in vitro using TNP-SRBC as antigen. By mixing irradiated spleen cells from low dose-primed mice with normal spleen cells, the enhancing activity is shown to be radiation resistant. Treatment with anti-BAϑ abolishes the activity. We therefore infer that it resides in a thymus-derived helper-cell population. While the enhancing activity is maximal for the carrier to which the mice were low dose primed, priming with SRBC gives a substantial enhancement of the response when burro RBC are used as carrier, but little enhancement when chicken RBC are used as carrier. There is little cross-reactivity between SRBC and burro RBC at the direct PFC level in this system, demonstrating that cross-reactivity at the T cell level need not parallel that at the B cell level.

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