Contact dermatitis presents as either allergic or irritant in etiology. While irritant contact dermatitis represents the direct toxic effect of an offending agent on the skin, allergic contact dermatitis (ACD) represents a delayed-type (type IV) hypersensitivity reaction that occurs when allergens activate antigen-specific T cells in a sensitized individual. Consequently, whereas irritant contact dermatitis can occur after one exposure to the offending agent, ACD typically requires repeat exposures before an allergic response is noted. ACD can occur 24-48 hours after exposure to the offending agent.
Contact dermatitis can demonstrate well-demarcated borders, suggestive of an "outside job" or external contact. The most common contact allergens are urishiol (poison ivy, oak, or sumac), nickel, fragrance, cobalt (a metal; see also cobalt toxicity), 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.
The distribution and geometry of lesions are important clues to diagnosis. It can also present as a systemic contact reaction with widespread lesions when the offending agent is ingested or present in an implanted device. ACD can occur in reaction to topical agents, ingested agents, implanted biomedical devices, and airborne materials.
American Contact Dermatitis Society Allergen of the Year:
- 2020 – Isobornyl acrylate, an acrylic monomer often used as an adhesive in medical devices. There have been multiple case reports of diabetes patients developing contact allergies to their insulin pumps. Acrylic nails are also a potential source. Clinician awareness is important because testing using routine panels does not identify isobornyl acrylate.
- 2021 – Acetophenone azine, thought to be formed in situ through reactions between ethyl vinyl acetate, which is present in foams used for cushioning in shin pads and footwear, and other additives. Reactions have been noted mostly in children and teens ("soccer shin" contact dermatitis).