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Micro & Immunology · single question

A 72-year-old patient has a positive PPD (tuberculin skin test). What does this indicate?

  1. Active pulmonary tuberculosis requiring immediate isolation
  2. Prior sensitization to TB antigens (Type IV hypersensitivity) — latent TB or prior BCG vaccination
  3. Immediate IgE-mediated allergy to tuberculin protein (Type I hypersensitivity)
  4. Current active TB that will definitely progress if untreated
Show answer and explanation

Correct answer: B. Prior sensitization to TB antigens (Type IV hypersensitivity) — latent TB or prior BCG vaccination

A positive PPD represents a Type IV (delayed-type, T-cell mediated) hypersensitivity reaction — it develops 48–72 hours after injection (delayed). It indicates prior sensitization to M. tuberculosis antigens from either: (1) latent TB infection, or (2) prior BCG vaccination. It does NOT confirm active disease. Chest X-ray and IGRA are needed for further workup.
  • A. ❌ Positive PPD alone doesn't diagnose active TB — symptoms, chest X-ray, and sputum tests are required.
  • B. ✅ Type IV (delayed-type) hypersensitivity reaction; indicates prior sensitization — latent TB or BCG vaccination.
  • C. ❌ Type I (IgE-mediated) = immediate allergic reaction (minutes) — PPD reaction takes 48–72 hours (delayed-type).
  • D. ❌ Positive PPD doesn't mean disease will progress — most latent TB never becomes active.

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