Athus reaction

• A local subacute type III hypersensitivity reaction.

• Typically caused by vaccination against tetanus and/or diphtheria.

Intradermal antigen injection in a presensitized individual (previously exposed to the antigen, with preformed, antigen-specific IgG in the serum) → formation of antigen-antibody complexes in the skin → complement activation → local inflammation and possibly necrosis

• Cutaneous small-vessel vasculitis [19]

• Localized swelling, erythema, hemorrhage

• Occasional superficial skin necrosis a few hours after booster vaccination

• The reaction typically peaks after 12–36 hours.

• Injection site reaction unrelated to hypersensitivity

• Abscess

• Cellulitis

• Shoulder injury related to vaccine administration

• Increased tetanus and diphtheria antibody levels in the serum support the diagnosis.

• Consider skin biopsy to rule out differential diagnoses.

• The reaction is self-limited.

• Symptomatic relief of swelling (e.g., cold compresses, NSAIDs, limb elevation)

Evidence of vasculitis on histology differentiates between a true Arthus reaction and an injection site reaction unrelated to hypersensitivity.