Allergic contact dermatitis is an inflammatory process induced by direct contact of the skin of a sensitized individual to an allergen. Within hours, red macules or patches develop that usually evolve rapidly, sometimes through an intermediate stage of urticarial papules or plaques, into vesicles that may become bullae. Because the lesions, i.e., macules, papules, vesicles, and bullae, are not in themselves specific, diagnosis clinically is made by virtue of distinctive distribution that reflects the manner in which a particular allergen was contacted.

Examples of common allergic contactants are Rhus toxicodendron ("poison ivy"), nickel, thiurames in rubber, parabens in ointments, and chemicals in fragrances, whereas agents commonly responsible for irritant contact dermatitis are acids, alkaloids, machine oils, organic solvents, and oxidants.