Delayed-type hypersensitivity (DTH) develops in the absence of an adjuvant when mice are injected intravenously or subcutaneously with an appropriate dose of sheep red blood cells (SRBC). The optimal intravenous dose of 105 SRBC (in CD-1 mice) produces maximum DTH which decays exponentially from its peak on day 4. Increasing the dose of SRBC reduces and eventually abolishes all evidence of DTH. DTH fails to reappear in respose to secondary stimulation except in splenectomized mice in whom the development of DTH is not suppressed, even by massive doses of SRBC. Hence the suppression cannot be due to antigen as such.

The optimal dose of SRBC for sensitization by footpad inoculation is 100-fold higher (107 SRBC in CD-1 mice), but even 109 SRBC do not block the induction of DTH by this route of immunization. A blocking dose of SRBC, given intravenously 1 day before footpad inoculation, completely suppresses cell proliferation in the draining lymph node, prevents PFC production there, and blocks the induction of DTH by a sensitizing dose of SRBC. If given 1 day after footpad sensitization, intravenous antigen has little effect on the cellular response in the regional node but DTH is still completely suppressed. Blocking of induction and expression may depend, therefore, on different mechanisms.

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