Contact dermatitis is probably the most commonly encountered immunologic disease by dermatologists. Prior to this described observation, few physicians recognized the concept of contact hypersensitivity. The field began to grow in the 1920s with further studies describing the phenomenon and has culminated in the modern era, present day, when volumes of information on the topic are available. Entire books are available listing compounds that have been noted to cause the disease process. An example is Fisher's Contact Dermatitis, a 1,117 page text that breaks down the subject by occupation, plants, and every other conceivable classification.
Contact dermatitis is due to either allergic reaction or, more commonly, irritant exposure.
Irritants usually are acids, alkalis, resins, or other chemicals, and frequently are found in drugs, dyes, plants, preservatives, cosmetics, and metals. Excessive moisture also may act as an irritant. Allergic contact dermatitis occurs in sensitized individuals through the mechanism of type IV, cell-mediated immunity. Re-exposure to sensitized antigens causes delayed hypersensitivity. Allergy and hypersensitivity do not play a role in irritant contact dermatitis, but an individual may develop an allergy to an irritant.