Which type of hypersensitivity reaction underlies the Arthus reaction and serum sickness?

• A Type I (IgE-mediated, immediate)
• B Type II (cytotoxic antibody-mediated)
• C Type III (immune complex-mediated) ✓
• D Type IV (delayed-type, T-cell mediated)

Explanation

Type III hypersensitivity (immune complex disease) involves the deposition of antigen-antibody complexes in tissues and vessel walls, activating complement and recruiting neutrophils via C3a and C5a. The Arthus reaction is a localised form (intradermal antigen in pre-sensitised host), while serum sickness is a systemic form occurring 7–14 days after antigen exposure. Distinguishing features: Type III involves IgG or IgM (not IgE), and complement plays a central role. Type II is directed against cell-surface antigens. Type IV is T-cell mediated and delayed (24–72 hours).

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

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