A 28-year-old veterinary worker with prior sensitization receives an intradermal injection of tetanus toxoid. Within 4 to 8 hours she develops marked edema and hemorrhagic induration at the injection site. Biopsy shows necrotizing vasculitis with abundant neutrophils and antigen-antibody complexes in vessel walls. This localized reaction is:

• A A delayed-type hypersensitivity response peaking at 48 hours
• B An Arthus reaction, a localized type III hypersensitivity response ✓
• C An immediate type I response mediated by preformed IgE
• D A type II reaction caused by antibodies against injected antigen on host cells

Explanation

The Arthus reaction is the experimental model of localized type III hypersensitivity: circulating IgG meets antigen deposited in vessel walls, forming immune complexes that activate complement and attract neutrophils, producing edema and hemorrhage within hours. The 4 to 8 hour onset separates it from type I reactions (minutes) and from classic delayed-type hypersensitivity, which peaks at 24 to 72 hours and is dominated by CD4 T cells and macrophages rather than neutrophilic vasculitis.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.