Abnormal cutaneous response to mitogens and a contact allergen in dogs with atopic dermatitis.

Antigen specific and nonspecific T-lymphocyte activity was evaluated in normal dogs and in dogs with atopic dermatitis by measuring the increase in skin thickness after application of the contact allergen dinitrochlorobenzene and after intradermal injection of the mitogens phytohemagglutinin and concanavalin A. The atopic dogs had a significantly reduced response to the contact allergen (P less than or equal to 0.001) but a significantly increased response to the mitogens (P less than or equal to 0.001). The atopic and normal dogs responded similarly to intradermally injected histamine. The response of dogs with non-atopic skin conditions to the cutaneous mitogen test was like that of normal dogs. Pre-existing dermatitis does not apparently influence cutaneous response to mitogens in dogs. The cutaneous response of atopics during treatment with corticosteroids is not different from normal controls. These results suggest a role for altered cell-mediated immunity in the pathogenesis of canine atopy and that the cutaneous mitogen test may have value as a rapid screening test for the disease.