Psoriasis vs allergic contact dermatitis in palms and soles: a quantitative histologic and immunohistochemical study

A systematic histologic and immunohistochemical study of cases of psoriasis (PSO) and allergic contact dermatitis (ACD) in palmo-plantar skin was performed to find differences between these two diseases that usually show overlapping features in these specific sites. Skin biopsies from 42 (22 female, 20 male) patients were evaluated for several histopathologic parameters and immunohistochemistry was applied to quantify keratinocytic proliferation, the number of dendritic cells (DCs) and the phenotype of the mononuclear cell infiltrate. Regular epidermal hyperplasia and marked parakeratosis were found to be more frequent in PSO than in ACD cases, but only the first parameter reached the level of significance (p = 0.03). The number of S100 protein-positive DCs was significantly higher in ACD (p = 0.006), whereas keratinocytic proliferation, studied with Mib-1, was found to be higher in PSO than in ACD, but the difference was not statistically significant. No significant difference was detected in the number of CD4+, CD8+ and bcl2+ lymphocytes in PSO and ACD cases. In the palms and soles, the finding of irregular epidermal hyperplasia and the detection of a higher number of S100 protein-positive DCs favor the diagnosis of ACD over PSO. The differential diagnosis between PSO and ACD can be practically approached using a histopathologic parameter and a commercially available antibody.