Which of the following events initiates T cell activation?

• A. Migration of T cells to the thymus
• B. Phagocytosis of pathogens by the T cell
• C. Release of cytokines by the T cell
• D. Binding of the T cell receptor (TCR) to an antigen-MHC complex on an antigen-presenting cell

Answer: (D)

Verified step by step guidance

1. Understand the role of T cells in the immune system: T cells are a type of lymphocyte that play a central role in adaptive immunity. They are activated when they recognize specific antigens presented by antigen-presenting cells (APCs).
2. Learn about the T cell receptor (TCR): TCRs are specialized proteins on the surface of T cells that bind to antigens. However, these antigens must be presented in a specific way, bound to major histocompatibility complex (MHC) molecules on APCs.
3. Explore the concept of antigen-MHC complexes: APCs, such as dendritic cells, macrophages, and B cells, process pathogens and display fragments of their antigens on MHC molecules. This presentation is crucial for T cell activation.
4. Understand the binding process: T cell activation begins when the TCR on a T cell binds specifically to the antigen-MHC complex on the surface of an APC. This interaction is highly specific and initiates a cascade of intracellular signaling events in the T cell.
5. Recognize the importance of co-stimulatory signals: In addition to the TCR-antigen-MHC binding, co-stimulatory signals provided by the APC are necessary for full T cell activation. These signals ensure that the immune response is appropriately regulated.