The TUBERCULIN (Mantoux) skin test is an example of which type of hypersensitivity reaction?

Correct answer: A. Type IV — delayed, cell-mediated

• A. CORRECT. The tuberculin skin test is the textbook example of TYPE IV DELAYED HYPERSENSITIVITY. Intradermal PPD is processed and presented to previously sensitised T LYMPHOCYTES, which release cytokines recruiting macrophages. The reaction is read at 48–72 HOURS and the measurement is of INDURATION, not erythema. There is no antibody and no complement involvement — it is purely cell-mediated, which is why the response is delayed.
• B. TYPE III involves deposition of antigen–antibody complexes with complement activation — serum sickness, SLE, post-streptococcal glomerulonephritis.
• C. TYPE I is IgE-mediated mast cell degranulation occurring within MINUTES — anaphylaxis, urticaria, allergic rhinitis, asthma.
• D. TYPE II involves IgG or IgM directed against cell-surface antigens causing cytotoxicity — transfusion reactions, autoimmune haemolytic anaemia, PEMPHIGUS VULGARIS and MUCOUS MEMBRANE PEMPHIGOID.

Note: Other Type IV examples worth recognising: CONTACT DERMATITIS (latex, nickel, and the delayed contact allergy occasionally seen to dental materials), graft rejection, and granuloma formation. The dental link is that latex allergy comes in both flavours — Type IV contact dermatitis (common, delayed) and Type I anaphylaxis (rare, immediate and dangerous).