Plant dermatitis. Possible culprits go far beyond poison ivy.

Given the variety of existing plant species in the environment, it is remarkable that people have adjusted as well as they have to the many plants that can cause uncomfortable skin reactions. With a basic understanding of the types of reaction and the common plants that cause each type, physicians can help patients discover the source of the dermatitis and thus prevent reexposure. In immediate contact dermatitis, welts form rapidly after patients brush against an offending plant, but the urticarial rash is short-lived. In irritant contact dermatitis, the skin is traumatized mechanically (eg, with cactus spines) or chemically (eg, with capsaicin from hot peppers), producing a more persistent skin reaction. Phytophotodermatitis occurs when the skin is exposed to sunlight after contact with an offending plant; reactions are erythema, pruritus, vesiculation, and subsequent hyperpigmentation. Allergic contact dermatitis, typified by the rash of poison ivy, is a cell-mediated immune response that occurs in previously sensitized persons. Erythema, vesiculation, and pruritus, which usually heal without causing pigmentary changes, may last for several weeks.