This summary discusses adult patients. Allergic contact dermatitis in children is addressed separately.

Allergic contact dermatitis (ACD) is a delayed-type (type IV) hypersensitivity reaction that occurs when allergens activate antigen-specific T cells in a sensitized individual. It typically requires repeated exposures before an allergic response is noted and most commonly occurs 24-48 hours after exposure to the offending agent.

Contact dermatitis generally presents with well-demarcated borders, suggestive of an "outside job" or external contact. Causative allergens and their clinical importance vary based on several factors, including exposures based on geographic regions, cultural practices, personal care product usage, and manufacturing practices (eg, which preservatives are used). Common contact allergens are urushiol (poison ivy, oak, and sumac), nickel, fragrance, cobalt, chromates (leather products), neomycin, thimerosal (ophthalmic preparations and vaccines), adhesives, and oxybenzone (sunscreens). Formaldehyde-releasing preservatives in polypropylene surgical masks have been reported to cause ACD.

Contact allergens may be airborne, such as from cement dust, sprays, or room fresheners, for example (airborne contact dermatitis).

Photoallergic contact dermatitis is contact dermatitis to an allergen that is incited by sun exposure. In North America, sunscreens are the most common photoallergen.

ACD can present as a systemic contact reaction with widespread lesions when the offending agent is ingested, present in an implanted device, or used in a manner that covers a large portion of the body (eg, body washes). Baboon syndrome (symmetric, sharply demarcated erythema of the gluteal or inguinal area as well as other intertriginous or flexural sites) has also been reported as a presentation of systemic ACD.

To raise awareness of increasingly common or problematic allergens, the American Contact Dermatitis Society selects an Allergen of the Year. Recent examples include sulfites (2024), lanolin (2023), and aluminum (2022).

Related topic: hand dermatitis, irritant contact dermatitis