Phenotypic difference between allergic and irritant patch test reactions in man.

Cellular responses in allergic and irritant contact dermatitis were analysed in situ using an immunohistochemical double staining technique with the aim of uncovering phenotypical differences of diagnostic importance. Allergic and irritant patch test reactions were elicited in 9 individuals using the Finn chamber technique. Thirty-nine skin biopsies from these reactions and from petrolatum controls were obtained 4 to 20 days after the test applications. Cell infiltrates were present throughout the observation period in both allergic and irritant reactions, but were usually greater in the former. In both types of reaction, anti-Leu 3a reactive cells predominated over anti-Leu 2a reactive cells. HLA-DR expression on keratinocytes was found in 9 of 14 allergic reactions, but not in irritant reactions or control areas. HLA-DQ antigens were not detected on keratinocytes. The presence of HLA-DR antigens on keratinocytes may reflect an immunological response of the allergic reactions, and thus be of diagnostic relevance.