A 32 year old man presents with a 3-month history of weight loss, night sweats, a productive cough with blood-tinged sputum, anorexia, general malaise, and a low grade fever. A PPD skin test shows > 10 mm of induration. If the area of induration were biopsied, which of the following type of reactive cells would be found?
**Core Concept**
The patient's symptoms and positive PPD skin test suggest an active tuberculosis (TB) infection. The PPD (purified protein derivative) test is a diagnostic tool for TB, which involves injecting a small amount of TB antigens into the skin. An induration (thickening of the skin) greater than 10 mm in diameter is considered a positive result, indicating exposure to TB.
**Why the Correct Answer is Right**
In response to the TB antigen, the body mounts a cell-mediated immune response, which involves the activation of macrophages and the recruitment of T lymphocytes to the site of antigen injection. The T cells, particularly the CD4+ helper T cells, play a crucial role in orchestrating the immune response against TB. These T cells release cytokines, such as interferon-gamma (IFN-γ), which activate macrophages to kill the TB bacilli. The activated macrophages and T cells accumulate at the site of antigen injection, causing the characteristic induration seen in a positive PPD test. The type of reactive cells found in the indurated area would be T lymphocytes, specifically CD4+ helper T cells.
**Why Each Wrong Option is Incorrect**
**Option A:** While neutrophils are present in various inflammatory conditions, they are not the primary type of reactive cells found in a positive PPD skin test.
**Option B:** Plasma cells are involved in the humoral immune response, producing antibodies against antigens. However, the PPD test is a cell-mediated immune response, and plasma cells are not the primary type of reactive cells found in this context.
**Option C:** Eosinophils are involved in allergic reactions and parasitic infections. They are not typically associated with the immune response to TB antigens.
**Clinical Pearl / High-Yield Fact**
The PPD skin test is a useful tool for diagnosing latent TB infection, but it has limitations. A false-negative result can occur in immunocompromised individuals or those with a recent TB infection. A false-positive result can occur in individuals with a history of BCG vaccination or other mycobacterial infections.
**Correct Answer: C. T lymphocytes (specifically CD4+ helper T cells)**