The T cells which mediate delayed-type hypersensitivity (DTH) to sheep red blood cells (SRBC) are blocked by a normal humoral response and cannot be made to function by further immunization. They can be rescued to some extent by treatment with immunopotentiating agents such as cyclophosphamide (CY) which suppresses the antibody response selectively, or by BCG which interferes with the action of serum blocking factors. These two agents together can restore cell-mediated immunity completely, but a further antigenic stimulus is needed to reestablish DTH in mice blocked by a long period of continuous exposure to SRBC.

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