During the first 3 months of infection of guinea pigs with living attenuated Pasteurella tularensis, the delayed hypersensitive response to killed organisms and the resistance of the animals to challenge with virulent organisms followed a parallel course. Guinea pigs immunized by vaccination with attenuated organisms were appreciably immune and highly hypersensitive by the end of the 1st month. Desensitization by daily injections with heat-killed organisms for 2 weeks reduced the hypersensitivity to a minimal level. When desensitization was discontinued reactivity gradually returned. Resumption of desensitization again caused a loss of reactivity. During the desensitizing procedures, immunity remained unchanged as determined by the mean survival time after challenge.

Immunization with heat-killed organisms in oil adjuvant by the intraperitoneal route produced a moderate degree of hypersensitivity after 3 weeks. These animals demonstrated, at most, a minimal level of acquired resistance. Immunization with specific antigen-antibody complex in oil adjuvant resulted in similar findings. By the 6th day after four simultaneous injections into the four footpads of guinea pigs, a moderate degree of hypersensitivity and a minimal level of resistance were evoked.

The lack of correlation between delayed hypersensitive and immune responses observed in this study indicates that these manifestations are probably independent immunologic phenomena, and it is concluded that the delayed hypersensitive response does not always reflect immune status.

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