Mice immunized with more SRBC than are required to produce optimal delayed-type hypersensitivity reactions, developed good antibody responses and poor delayed foot pad reactions. Cyclophosphamide treatment in low doses (20 mg/kg) before immunization, augmented the delayed-type hypersensitivity without affecting antibody responses. Cyclophosphamide did not augment delayed responses to optimal doses of SRBC (0.01%), but did augment the delayed hypersensitivity response of mice immunized with a suboptimal antigen dose (0.001%); which produced no detectable antibody response with or without cyclophosphamide pretreatment. These results suggest that antibody feedback is not the sole regulator of delayed reactions; the possibility that suppressor T cells may also be involved is discussed.

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