**Core Concept:** Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis, primarily affecting the lungs. The tuberculin skin test (TST) is a diagnostic tool used to detect latent TB infection (LTBI) or previous exposure to the bacteria. The test involves injecting purified protein derivative (PPD) into the skin, which stimulates a delayed-type hypersensitivity response through activation of helper T cells.

**Why the Correct Answer is Right:** In a positive tuberculin skin test, helper T cells (specifically, CD4+ T cells) play a crucial role in the immune response against M. tuberculosis. These T cells help coordinate the immune response by activating macrophages, which are crucial for phagocytosis and subsequent destruction of the bacteria. Additionally, they stimulate B cells to produce antibodies and promote the activation of cytotoxic T cells to eliminate infected cells.

**Why Each Wrong Option is Incorrect:**

A. **Does not involve helper T cells:** This option is incorrect because helper T cells are essential in the immune response against TB. They directly participate in activating other immune cells needed for controlling M. tuberculosis infection.

B. **Does not coordinate immune response:** This option is incorrect because helper T cells coordinate the immune response by activating macrophages, B cells, and cytotoxic T cells, which are vital for combating TB infection.

C. **Does not stimulate B cells:** This option is incorrect because helper T cells stimulate B cells to produce antibodies that help neutralize M. tuberculosis and contribute to the overall immune defense against TB infection.

D. **Does not involve cytotoxic T cells:** This option is incorrect because helper T cells activate cytotoxic T cells, which are crucial for eliminating infected cells and preventing TB progression.

**Clinical Pearl:** A positive TST result indicates the presence of LTBI, which can progress to active TB disease if the immune system fails or is suppressed. The test result should be interpreted in conjunction with clinical and epidemiological factors to make an accurate diagnosis and guide further management, such as starting chemoprophylaxis or conducting a chest X-ray.