A patient was injected with Purified Protein Derivative (PPD) to diagnose TB disease or a previous exposure to Mycobacterium tuberculosis. The injected area, when inspected after 48-72 hours, was found to develop induration (thick, hardened bulge). Which of the following cells will be predominant at this site?

Correct option is B

The PPD (Purified Protein Derivative) test, also known as the Tuberculin Skin Test (TST) or Mantoux test, is used to detect exposure to Mycobacterium tuberculosis. The immune response to PPD is a delayed-type hypersensitivity (DTH) reaction, which occurs 48-72 hours after injection.

Key Immunological Mechanism:

• T-helper cells (CD4+ T cells) recognize tuberculin antigens and release cytokines (IFN-γ, TNF-α).
• Macrophages are recruited to the site, leading to inflammation and induration (hardened skin swelling).

Since T-helper cells and macrophages are the primary immune cells involved in delayed-type hypersensitivity (Type IV hypersensitivity), option (2) is correct.

Information Booster

1. Type IV Hypersensitivity (DTH Reaction)

   • Mediated by T-helper cells (CD4+ Th1 cells).
   • Does not involve antibodies (unlike Type I, II, or III hypersensitivity).
   • Requires 48-72 hours for response.

2. Macrophage Activation

   • Macrophages secrete pro-inflammatory cytokines (IL-1, TNF-α, IFN-γ).
   • Leads to localized inflammation and induration at the injection site.

3. Induration vs. Redness:

   • Induration (hard swelling) indicates a positive reaction (prior exposure to TB).
   • Redness alone is not a positive result.

4. Clinical Significance of PPD Test

   • Positive Result: Indicates past TB infection or latent TB (not necessarily active TB).
   • Negative Result: No exposure or severely weakened immunity (e.g., in HIV/AIDS).

Additional Information

• Option (1) Neutrophils and Mast Cells (Incorrect)

  • Neutrophils are first responders in acute infections but not key players in Type IV hypersensitivity.
  • Mast cells are more relevant in Type I hypersensitivity (allergic reactions, histamine release).

• Option (3) Eosinophils and Mast Cells (Incorrect)

  • Eosinophils are involved in parasitic infections and allergies, not TB immunity.
  • Mast cells are part of immediate hypersensitivity reactions.

• Option (4) Natural Killer (NK) Cells and Dendritic Cells (Incorrect)

  • NK cells are part of the innate immune response (not involved in DTH).
  • Dendritic cells play a role in antigen presentation but do not dominate the PPD response.