Comparison of Allergic and Irritant Contact Dermatitis

Abstract

The distinction between irritant and allergic contact dermatitis has become increasingly blurred. At present, the difference can only be demonstrated in extreme examples, and disappears with milder reac- tions. Mathias and Maibach defined irritant contact dermatitis as nonimmunologic local inflammatory reaction characterized by erythema, edema, or corrosion following single or repeated application of a chemi- cal substance to an identical cutaneous site.y If the word corrosion is omitted, as it represents the extreme situation, and the word nonimmu- nologic is changed to immunologic, we have a definition of allergic contact dermatitis. The distinction between an immunologic and nonim- munologic mechanism is more conceptual than demonstrable, as will be seen in this review of recent findings in irritant contact dermatitis. The appropriate diagnosis for any given patient is usually deter- mined by a combination of morphologic, histologic, and immunologic findings, and the time course of the dermatitis. Morphologic guidelines for distinguishing the two diseases have been offered,' but are not useful for mild to moderate reactions. Histologic differentiation is rarely possible.8 Patch test results are helpful but do not eliminate the possible coexistence of both diseases. A negative patch test can also represent not testing the correct substance or the correct concentration. (See Figure 1.) Immunologically mediated inflammation requires a period of induc-