Balb/c spleen cells from keyhole limpet hemocyanin (KLH) primed mice exposed to picogram quantities of KLH for 15 min in vitro became susceptible to inactivation by antiimmunoglobulin and complement. Susceptibility to antiimmunoglobulin and complement was detected as the loss of helper function measured by the inability of such cells to cooperate in an in vitro antihapten response to trinitrophenyl-KLH. Helper cells to chicken gamma globulin in the same mixture were not affected indicating specificity of the effect. The induction of susceptibility was temperature dependent; it could be induced at 37° or 30° but not at 4°. Cells became maximally susceptible to antiimmunoglobulin and complement within 15 min after exposure to the low dose of carrier after which susceptibility was lost indicating that the induced state was transient. A model is proposed for low dose dependent arming of helper cells which would permit them to discriminate between antigen challenge and background noise.
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