Final answer:
The complementarity-determining regions (CDR) 1 and CDR2 loops of the T-cell receptor primarily interact with major histocompatibility complex (MHC) molecules presenting processed antigens, which is crucial for T-cell activation and the adaptive immune response.
Step-by-step explanation:
The complementarity-determining region (CDR) 1 and CDR2 loops of the T-cell receptor contact the major histocompatibility complex (MHC) molecules that present processed antigens.
Each T-cell receptor (TCR) consists of two chains (α and β chains) that contain both constant and variable regions. The variable region of each chain incorporates three hypervariable or complementarity-determining regions (CDRs), which recognize processed antigens. Specifically, the CDR1 and CDR2 loops, along with the CDR3 loop of both the α and β chains, contribute to the formation of the antigen-binding site.
This site directly interacts with the complex of antigen peptides bound to MHC molecules on the surfaces of antigen-presenting cells (APCs). The MHC molecule presents the antigen in a form that the TCR can recognize, facilitating T-cell activation. The interaction between the TCR and MHC-antigen complex is essential for the adaptive immune response, as each T cell is typically specific to a single antigen, thanks to the unique combination of amino acids in its TCR variable domains.