ABSTRACT
In 1898, contact dermatitis was fi rst appreciated to have more than one mechanism, and is now generally divided into irritant contact dermatitis (ICD) and allergic contact dermatitis (ACD), on the basis of these mechanistic differences. ACD is a delayed (type IV) hypersensitivity reaction, mediated by T cells and requiring prior sensitization, whereas ICD has a nonimmunologic mechanism, thus not requiring sensitization. Clinical distinction of the two processes is often challenging, as morphology and histopathology of irritant and allergic dermatitis reactions can be virtually indistinguishable. The two processes may, and often do, coexist, thereby further complicating matters.
