Allergic contact dermatitis - Hair and Scalp
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). 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.
L23.9 – Allergic contact dermatitis, unspecified cause
40275004 – Contact dermatitis
Differential Diagnosis & Pitfalls
Drug Reaction Data