A sensitized laboratory worker receives an intradermal injection of a protein antigen he has been exposed to repeatedly. Within 4 hours the site shows marked edema, hemorrhage, and ulceration. Histology of the lesion would most likely show dense infiltration by which cell type, and what is the hypersensitivity classification?

• A CD4+ Th1 lymphocytes; type IV (delayed type)
• B Neutrophils; type III (immune complex mediated) ✓
• C Mast cells and eosinophils; type I (immediate)
• D Cytotoxic CD8+ T cells; type II (antibody dependent)

Explanation

The Arthus reaction is a localized type III reaction: antigen injected into a previously sensitized host forms immune complexes in vessel walls, fixes complement, and attracts neutrophils via C5a, producing edema, hemorrhage, and necrosis within hours. The tempting distractor is type IV, because the onset seems delayed, but true delayed reactions peak at 24 to 72 hours and are dominated by T cells and macrophages rather than neutrophilic vasculitis.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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