What do T cells require in order to become activated?

T cells require binding to antigen-presenting cells (APCs) in order to become activated. This activation process is crucial for the immune response. When a T cell encounters an APC, such as a dendritic cell or a macrophage, it interacts with the major histocompatibility complex (MHC) molecules displayed on the surface of the APC. The MHC molecules present processed antigens, which are specific to the pathogen that the immune system is trying to target.

Once the T cell receptor (TCR) on the T cell binds to the antigen-MHC complex on the APC, this initiates a cascade of intracellular signals that lead to T cell activation. In addition to the direct binding, a second signal, often from co-stimulatory molecules also present on the APC, is necessary for full activation. This dual-signal requirement ensures that T cells do not respond indiscriminately and helps maintain immune system homeostasis.

The other options pertain to different aspects of immune function but are not directly involved in T cell activation. Exposure to antibodies does not activate T cells; rather, it is primarily a part of the humoral immune response and influences B cells. The release of cytokines generally supports and modulates the immune response but