Type IV Hypersensitivity — MFDS Part 1 MCQ

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ModeratePhysiologyType IV HypersensitivityMFDS Part 1

A patch-test site becomes indurated and eczematous 48 hours after application of a nickel allergen. Which cells are the principal antigen-specific effectors responsible for this type IV hypersensitivity reaction?

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Correct answer: C — Activated antigen-specific T lymphocytes

Type IV hypersensitivity is mediated by activated, antigen-specific T lymphocytes rather than antibodies. CD4+ helper T cells can release cytokines that recruit and activate macrophages and other inflammatory cells, while CD8+ cytotoxic T cells may directly injure target cells; their relative contributions vary between type IV subtypes. The delayed 48-hour patch-test response is characteristic. Mast-cell degranulation is central to immediate, IgE-mediated type I hypersensitivity. B lymphocytes and their antibodies principally underlie humoral reactions, including types II and III. Eosinophils and neutrophils can be recruited in particular type IV patterns, but they are not the defining antigen-specific cells initiating the response.

Reference: Marwa K, Goldin J, Kondamudi NP. Type IV Hypersensitivity Reaction. StatPearls, updated 4 May 2025 (2026 edition). https://pubmed.ncbi.nlm.nih.gov/32965899/