Basic and Clinical Sciences Examination (BCSE) Practice Exam 2025 – Comprehensive All-in-One Guide for Exam Success

The tuberculin skin test (TST) is a classic example of a type IV hypersensitivity reaction, also known as delayed-type hypersensitivity. This immune response is primarily mediated by T cells rather than antibodies, and it is characterized by the activation of helper T cells that can lead to the release of various cytokines.

In the context of the TST, the presentation of the tuberculin antigen (a protein derived from Mycobacterium tuberculosis) to sensitized helper T cells results in the release of specific cytokines, notably interferon-gamma (IFN-gamma). This cytokine is key in orchestrating the immune response, enhancing macrophage activation, and promoting inflammation at the site of the antigen exposure. The release of IFN-gamma leads to the recruitment of additional immune cells, including macrophages, which contribute to the observable induration and erythema in the skin at the site of the injection.

The other options, while involved in various immune responses, do not accurately represent what triggers the type IV hypersensitivity reaction in the TST. For example, IL-6 is more associated with acute inflammatory responses and systemic immune responses. The presence of IgE antibodies is related to type I hypersensitivity reactions, such as