Cell-mediated immunity includes both the generation of cytotoxic cells and initiation of delayed-type hypersensitivity (DTH). The resting T-cell population, before stimulation by antigen, already contains cells of the Lyl subclass that are programmed to initiate DTH (and helper function) but not cytotoxic responses, as well as Ly23 cells which can generate killer activity (and suppressive function) but not DTH. The central implication of these findings is that the broad division between humoral and cell-mediated immune responses does not precisely correspond to the division of labor among T-cell subclasses. The relative contribution of DTH-competent Lyl cells and cytotoxic Ly23 cells to the classical homograft response remains to be determined.

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