Immunohistochemistry of lymphocytes and Langerhans' cells in long-lasting allergic patch tests.

A long-lasting allergic patch test is a "normal" allergic patch test that remains positive for weeks or months. An immunohistochemical study of immunocompetent cells in the skin in this rare type of patch tests was performed. Most inflammatory cells were T11 positive T-lymphocytes. The majority of these cells were of the helper/inducer phenotype (T4+), but a relative increase of T8+ cells as compared to the initial (1-2d) stages of allergic patch tests was observed. T6+ Langerhans' cells (LCs) were normal or increased in number in the epidermis, while very few dendritic cells displayed Ial antigen in the epidermis, indicating loss of Ial-staining of LCs. High to very high numbers of T6+ cells were found in the dermis. An inflammatory reaction of hair follicles with moderate numbers of T6+ cells in the peribulbar infiltrate was observed indicating that hair follicles might act as shunt pathways for allergens. A defect in down regulation of the contact hypersensitivity reaction and/or a constant antigen stimulation could be responsible for the long-lasting allergic patch tests.