In the Arthus reaction, a farmer develops erythema and induration at the site of repeated subcutaneous antigen injection after 4–8 hours. The primary mediators are:

• A IgE and mast cell tryptase
• B Sensitized CD4+ T cells releasing IFN-γ
• C IgM-mediated complement lysis of local endothelium
• D IgG–antigen immune complexes activating complement and recruiting neutrophils ✓

Explanation

The Arthus reaction is a local Type III (immune complex) hypersensitivity where preformed high-titer IgG complexes precipitate in vessel walls, activate complement (C3a, C5a), and recruit neutrophils causing vasculitis. IgE/mast cell mediators cause Type I reactions within minutes. CD4+ T cells releasing IFN-γ mediate Type IV delayed hypersensitivity. IgM-mediated lysis is the mechanism of Type II cytotoxic hypersensitivity.

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

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