During investigation of a hypersensitivity reaction, intradermal injection of an antigen produces induration and erythema peaking at 48-72 hours. Histological examination of the reaction site shows predominantly CD4+ T lymphocytes and activated macrophages without significant eosinophils. This pattern is most consistent with which Coombs and Gell classification?

• A Type I (IgE-mediated)
• B Type III (immune complex-mediated)
• C Type II (cytotoxic)
• D Type IV (delayed-type hypersensitivity) ✓

Explanation

Type IV hypersensitivity (delayed-type) is T-cell mediated; it peaks at 48-72 hours rather than within minutes (Type I) or a few hours (Type III Arthus reaction). The infiltrate consists of CD4+ Th1 cells secreting IFN-γ that activate macrophages, producing the characteristic indurated lesion. Type I involves mast cell degranulation with IgE. Type III involves immune complex deposition and complement activation with neutrophil-predominant inflammation.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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