Allergic Contact Dermatitis — SCE Dermatology MCQ

Correct answer: A — Type IV (delayed/cell-mediated)

The correct answer is A, Type IV (delayed/cell-mediated) hypersensitivity. This patient has allergic contact dermatitis to potassium dichromate, a chromate salt used in leather tanning, with the eruption confined to the dorsal feet matching the sandal contact area and a positive patch test confirming the allergen. Allergic contact dermatitis is a delayed-type reaction mediated by allergen-specific T lymphocytes, requiring an initial sensitisation phase followed by an elicitation phase on re-exposure, with clinical changes typically peaking 48 to 72 hours after contact rather than occurring immediately. Patch testing itself works precisely because it reproduces this T cell mediated response, with reactions read at delayed timepoints (typically day 2 and day 4 to 7), which is diagnostic of a type IV mechanism rather than an antibody mediated process. Why the other options are wrong: E. Type III (immune complex): This involves antigen-antibody complex deposition causing conditions like vasculitis or serum sickness, not a localised eczematous eruption at a site of direct skin contact with an external chemical. D. Type II (cytotoxic): This is antibody mediated destruction of cells bearing a specific antigen, such as in haemolytic disease or Goodpasture syndrome, and does not explain a T cell driven dermatitis confined to a contact site. C. Type I (immediate): This is IgE mediated mast cell degranulation producing urticaria, angioedema or anaphylaxis within minutes, whereas this presentation developed after cumulative sandal exposure with a delayed course, not an immediate wheal and flare. B. Type V (stimulatory): This is not a standard immunopathological category in the Gell and Coombs classification used clinically; some texts describe antibody mediated receptor stimulation or blockade (e.g. Graves disease), which is irrelevant here. Key point: Allergic contact dermatitis from chromate sensitised leather is the classic exam example of a type IV, T cell mediated, delayed hypersensitivity reaction, confirmed by patch testing.

Reference: Predictive factors for Day 7 positive patch test readings at a secondary referral centre, Skin Health and Disease, PMC, 2022: Allergic contact dermatitis (ACD) is caused by delayed-type allergy (type IV hypersensitivity) to contact allergens. https://pmc.ncbi.nlm.nih.gov/articles/PMC9060153/