How do antigen-presenting cells activate T cells?

• A. By secreting antibodies directly
• B. By presenting antigens on MHC molecules
• C. By causing inflammation
• D. By destroying infected cells

Answer: (B)

Antigen-presenting cells (APCs) activate T cells primarily through the process of presenting antigens on Major Histocompatibility Complex (MHC) molecules. When an APC, such as a dendritic cell, macrophage, or B cell, encounters a pathogen, it processes the pathogen's proteins into peptide fragments. These peptides are then loaded onto MHC molecules, which are expressed on the surface of the APC.

When a T cell encounters an APC, it recognizes the specific peptide-MHC complex through its T cell receptor (TCR). This recognition is crucial for T cell activation, as it guarantees that the T cell is responding to a relevant pathogen. The interaction between the TCR and the peptide-MHC complex initiates a series of signaling events within the T cell, leading to its activation, proliferation, and differentiation into effector or memory T cells.

Additionally, the activation of T cells is often accompanied by co-stimulatory signals from the APC, which are necessary for a full immune response. This multifaceted engagement ensures that T cells only become activated in response to actual threats, contributing to a robust and specific immune response. While inflammation and the destruction of infected cells are vital components of the immune response, they are not the primary