Final answer:
The antigen-recognition site of T-cell receptors is formed by the association of the variable domains from both chains of the receptor, which determine antigenic specificity. The correct answer is option 1.
Step-by-step explanation:
T-cell receptors (TCRs) are glycoprotein molecules found on the surface of T lymphocytes, and they play a crucial role in the adaptive immune response by recognizing specific antigens presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells (APCs).
TCRs consist of two distinct chains: the alpha (α) chain and the beta (β) chain, which together form the heterodimeric structure of the TCR. Each chain consists of constant (C) and variable (V) domains.
The variable domains of the TCRs are responsible for antigen recognition. These variable domains contain hypervariable regions, also known as complementarity-determining regions (CDRs). These CDRs specifically interact with the antigenic peptides presented by MHC molecules. The amino acid sequences within the CDRs vary significantly among different TCRs, allowing for diverse antigen recognition.
While both variable and constant domains are present within the TCR structure, the antigen-recognition site, crucial for interacting with specific antigens, is primarily formed by the variable domains due to their hypervariable regions, which confer specificity to antigen recognition. Constant domains provide structural stability and are not directly involved in antigen recognition.