Contact dermatitis: clinics and pathology

Contact dermatitis or eczema is a polymorphic inflammation of the skin. It occurs at the site of contact with irritating or antigenic substances. In t he acute phase there is occurrence of itching erythema, papules, and vesicl es, whereas in the chronic phase there is dryness, hyperkeratosis, and some times fissures. Contact dermatitis can be divided into irritant and allergi c types. Allergic contact dermatitis is a type-IV T-cell-mediated reaction occurring in a sensitized individual after contact with the antigen/allerge n. Such antigens are usually low molecular weight substances (MW similar to 500), called haptens; 3000 contact allergens are known. The diagnosis of c ontact allergy is made on the basis of the history, clinical findings, and a positive epicutaneous test result. Allergic, but not irritative, contact dermatitis can spread beyond the area of contact to other body parts. Eczem atous lesions are characterized by a mononuclear infiltrate consisting main ly of T cells in the dermis and epidermis, together vith an intercellular e pidermal edema-that is, spongiosis. In allergic contact dermatitis, skin-ap plied antigen is taken up by epidermal Langerhans cells and transported wit h the afferent lymph to the regional lymph nodes. Here, naive T lymphocytes are sensitized to become antigen-specific effector T cells, which then lea ve the lymph node, enter the circulation, and are recruited to the skin by means of specific cell surface molecules, to form the infiltrates. Cytokine s released by infiltrating T cells eventually cause keratinocyte apoptosis.