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.

ACD 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 certain geographic regions, cultural practices, personal care product usage, and manufacturing practices (ie, 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).

ACD can also 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). ACD can occur in reaction to topical or ingested agents, implanted biomedical devices, and airborne materials.

The intense erythema and sharply demarcated plaques of contact dermatitis are easy to confuse with cellulitis or erysipelas, especially when vesiculation and bullae formation take place. Differentiating features include the presence of pruritus (often extreme) in contact dermatitis versus the skin warmth and spreading erythema of a soft tissue infection. A detailed allergen exposure history should be elicited.