T cells play a critical role in adaptive immunity by recognizing and responding to specific antigens. T cells do this by using their T cell receptors (TCRs) to recognize antigenic peptides that are presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells (APCs).
MHC molecules are cell surface proteins that present antigenic peptides to T cells. There are two classes of MHC molecules: class I and class II. Class I MHC molecules are expressed on all nucleated cells and present peptides derived from intracellular proteins, such as viral proteins or proteins from cancer cells. Class II MHC molecules are expressed on APCs, such as dendritic cells, macrophages, and B cells, and present peptides derived from extracellular proteins, such as bacterial proteins.
T cells can only recognize antigenic peptides that are presented in the context of MHC molecules. This is because the TCR recognizes a complex of the antigenic peptide and the MHC molecule, rather than the antigenic peptide alone. The TCR is a heterodimeric protein that consists of an α and a β chain, or a γ and a δ chain. The variable regions of the TCRs, known as the hypervariable complementarity-determining regions (CDRs), are responsible for recognizing the antigenic peptide-MHC complex.
The interaction between the TCR and the antigenic peptide-MHC complex is a key event in T cell activation. When a T cell recognizes an antigenic peptide-MHC complex that it recognizes, it becomes activated and undergoes clonal expansion, producing many copies of itself that can respond to the pathogen. Activated T cells can then differentiate into effector T cells, which have various functions, including killing infected cells, activating other immune cells, and producing cytokines.
The recognition of antigenic peptides by TCRs is a highly specific process, and T cells can recognize even small differences in antigenic peptides that are presented by different MHC molecules. This specificity is critical for the ability of the adaptive immune system to respond to a wide variety of pathogens.