Type IV hypersensitivity
USMLE Step 1Difficulty: easyLast updated:
A 32-year-old woman presents with an intensely pruritic, eczematous, vesicular plaque on her lower abdomen exactly beneath the metal button of her jeans. She has had similar reactions on her earlobes when wearing inexpensive earrings. Patch testing with nickel sulfate produces an erythematous, papulovesicular reaction at the test site when read 2 days after application.
Which of the following best describes the mechanism of this reaction?
Answer and explanation
Correct answer: B. Previously sensitized T cells responding to antigen presented in the skin
A delayed eczematous reaction at a site of metal contact, a history of similar reactions to earrings, and a positive nickel patch test indicate allergic contact dermatitis. The source describes allergic contact dermatitis as a type IV, T-cell–mediated, delayed-type hypersensitivity reaction with two phases. In the sensitization phase, Langerhans cells capture the allergen, migrate to regional lymph nodes, and present it to naive T cells, which expand into memory and effector T cells. On re-exposure (the elicitation phase), antigen-presenting cells present the antigen to sensitized T cells in the skin, producing the inflammatory reaction. The source notes that nickel sulfate is the most common contact allergen in most populations. Irritant contact dermatitis, in contrast, is direct tissue damage that does not require prior sensitization.
Each option explained
- A. Deposition of antigen–antibody complexes in dermal vesselsIncorrect. Immune complex deposition (type III) causes vasculitis or serum sickness, not a localized eczematous plaque at a contact site.
- B. Previously sensitized T cells responding to antigen presented in the skinCorrect. Allergic contact dermatitis is a type IV, T-cell–mediated delayed-type hypersensitivity reaction.
- C. Direct chemical damage to the epidermis without prior sensitizationIncorrect. This describes irritant contact dermatitis; a positive patch test and recurrent reactions to nickel indicate immune sensitization.
- D. Cross-linking of allergen-specific IgE on mast cellsIncorrect. IgE cross-linking causes type I (immediate) reactions such as urticaria, which appear within minutes.
- E. IgG antibodies binding to antigens on keratinocyte surfacesIncorrect. IgG against epidermal surface antigens is a type II mechanism (e.g., pemphigus vulgaris).
Key point
Allergic contact dermatitis (e.g., nickel) is a type IV, T-cell–mediated delayed-type hypersensitivity reaction confirmed by patch testing.
From the source
“ACD is a type IV, T-cell–mediated, delayed-type hypersensitivity reaction to an environmental allergen that has 2 phases: Sensitization to an antigen Allergic response after reexposure”