The Mantoux test uses 5TU of PPD (purified protein derivative) intradermally. In a patient recently vaccinated with BCG, a 12 mm induration is read at 48–72 hours. Which immunological mechanism underpins this reaction?

• A Type I (IgE-mediated) hypersensitivity to PPD
• B Type III (immune complex) hypersensitivity causing local vasculitis
• C Type II (antibody-mediated cytotoxicity) by anti-mycobacterial IgG
• D Type IV (delayed-type, cell-mediated) hypersensitivity mediated by sensitized CD4+ Th1 cells and macrophages ✓

Explanation

The Mantoux reaction is a classic example of Type IV (delayed-type) hypersensitivity. Memory CD4+ Th1 T cells sensitized by prior exposure to mycobacterial antigens (infection or BCG vaccination) recognize PPD antigens presented by local APCs, release IFN-γ and IL-2, and recruit macrophages, producing induration (>10mm) at 48–72 hours. There is no IgE, complement, or antibody involvement. A positive reaction indicates sensitization, not active disease.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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