Although the concept, that hypersensitivity plays an important role in the development of the lesion of primary local vaccinia, was offered by yon Pirquet (1) early in this century, it has been obscured in recent years by greater interest in the lytic abilities of the multiplying virus in host cells, especially as observed in tissue culture (2-4).However, when opposing hypotheses of lesion formation, i.e. the allergic ~ the cytopathic concepts, have been put to test in intact animals, the experimental results have favored hypersensitivity over cytoI~thogenicity as basically responsible for the skin lesion. Thus, in children the shortened incubation period of a second, local vaceinial lesion derived from an infection initiated 2 days after a primary inoculation of vaccinia virus, indicated:that the second infection was not an independent event, but rather was influenced by allergy already provoked by the first infection in the same host (5). Pincus and Flick (6) also found that a primary vaccinial lesion in the guinea pig was completely suppressed by the local injection of a rabbit antiguinea pig mononudear ~ serum. This treatment also prevented the occurrence of delayed hypersensitivity to inactivated vaccinia virus as well as other expressions of delayed allergy.But before the role of developing hypersensitivity in the production of the vesicular lesion of primary vaccinia can be considered as proven, further evidence supporting this concept must be obtained by other, unrelated technics. Accordingly, this report deals with the suppression of the primary vaccinial lesion when delayed hypersensitivity to the vacduial antigens is inhibited by another method. The method of immunological tolerance was used in newborn rabbits to suppress the development of allergy to the vaccinial antigens. This procedure also accomplished a suppression of the expected local lesion following infection with the active virus.The use of microbial antigens to induce specific immunological tolerance has lacked the degree of success associated with tolerance directed toward tissue homo-*
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