Final answer:
T cells are activated by antigens presented by APCs such as macrophages or dendritic cells. The correct answer is the thymus gland.
Step-by-step explanation:
T cells are a type of lymphocyte that play a crucial role in the immune response. In order to be completely functional, T cells must be activated by antigens, which are molecules found on the surface of pathogens. Antigens are typically presented to T cells by antigen-presenting cells (APCs) such as macrophages or dendritic cells.
The process of T cell activation involves the binding of an antigen to the T cell receptor (TCR) on the surface of the T cell. This binding is specific, meaning that each T cell receptor can only recognize a particular antigen. Once the TCR binds to the antigen, signaling pathways are activated within the T cell, leading to its activation and proliferation.
Therefore, the correct answer is E) the thymus gland, as T cells mature in the thymus and are activated by antigens.