Which statement characterizes contact dermatitis?

• A. Caused by preformed IgE antibody
• B. Characterized by infiltration of neutrophils into the site of the reaction
• C. The primary symptoms often occur in the respiratory tract
• D. Usually due to a hapten

Answer: (D)

Contact dermatitis is a delayed-type hypersensitivity reaction in the skin driven by T cells. Many contact allergens are haptens—small molecules that are not antigenic by themselves but become antigenic when they bind to skin proteins to form hapten-protein complexes. These complexes are processed by antigen-presenting cells and presented to T cells, leading to sensitization. On re-exposure, memory T cells release cytokines that recruit inflammatory cells and cause the vesicular, itchy dermatitis at the exposure site. This hapten-driven, T-cell–mediated process, rather than IgE-mediated or neutrophil-dominated mechanisms, is what makes the statement that it is usually due to a hapten the best description. The other ideas—preformed IgE antibodies, primary neutrophil infiltration, or symptoms in the respiratory tract—do not fit contact dermatitis.