Final answer:
T cells refer to T Lymphocytes, which are essential components of the immune system, including helper, cytotoxic, and regulatory T cells, each playing specific roles in immune response regulation and pathogen defense.
Step-by-step explanation:
The T cells referred to in the question are a type of white blood cell that plays a central role in the immune response. The correct answer to the question 'What are T cells?' is D) T Lymphocytes. T lymphocytes come in various forms, including helper T cells (TH), cytotoxic T cells (TC or CTLs), and regulatory T cells (Treg). They have different functions in the immune system, such as aiding in the activation of other immune cells, directly attacking infected cells, and regulating immune responses to prevent overactivity.
Helper T cells are crucial in determining the specific immune response that will be developed, guiding the actions of other immune cells. In contrast, cytotoxic T cells target and destroy cells that are infected by pathogens or are otherwise dysfunctional. Overall, understanding the various roles of T cells is essential for a comprehensive understanding of the immune system's function and the pathogenesis of immune-related diseases.