Final answer:
T cell receptors (TCRs) initiate an adaptive immune response by binding to antigens when they are presented on the surface of cells by Major Histocompatibility Complex (MHC) molecules. They contribute to the specificity of the immune system through their variable domains, allowing for a vast array of unique TCRs suitable for different antigens.
Step-by-step explanation:
The student asked about the initiation of an adaptive immune response by T cell receptors (TCRs). TCRs are crucial components of the adaptive immune system, providing the specificity needed to target a wide array of antigens that an organism might encounter throughout life. A TCR is comprised of two polypeptide chains, each with a constant and a variable domain. The variable domain allows for the enormous diversity of TCRs necessary to recognize different antigens. T cells express thousands of identical copies of a specific TCR variant on their cell surface.
TCRs initiate an adaptive immune response primarily by binding only when antigens are presented via Major Histocompatibility Complex (MHC) molecules. When a TCR binds to the complementing antigen-MHC complex, the T cell becomes activated, which starts a cascade of immune responses, including clonal expansion. This process signifies the adaptive immune system's need to activate a specific T cell population with complementary TCRs to the invading pathogen. TCR diversity is achieved through gene recombination and mutation during T cell development. Clonal selection ensures that only T cells with receptors specific to the antigen in question proliferate, forming an army of T cells all expressing the same antigen-specific TCR.