Which parts of the T-cell antigen receptor account for the diversity of antigens that T-cells can recognize?
The correct answer and explanation is:
Correct Answer:
The variable (V) regions of the alpha (α) and beta (β) chains
Explanation:
The diversity of antigens that T-cells can recognize is primarily due to the variable (V) regions of the alpha (α) and beta (β) chains of the T-cell antigen receptor (TCR). Each TCR is composed of two polypeptide chains, usually an alpha chain and a beta chain. These chains have both constant (C) and variable (V) regions. The variable regions are the parts that directly interact with antigens, and they are responsible for the receptor’s specificity.
The generation of diversity in the variable regions occurs during T-cell development in the thymus. This diversity is created through a process called V(D)J recombination, which randomly rearranges gene segments known as variable (V), diversity (D), and joining (J) segments. The alpha chain is formed by rearranging V and J segments, while the beta chain is formed by rearranging V, D, and J segments. This random recombination, combined with additional nucleotide insertions or deletions at the joining sites, creates a vast number of unique TCRs.
Another source of diversity is the pairing of different alpha and beta chains, which further increases the number of possible TCR combinations. This high variability allows the immune system to recognize a broad range of antigens presented by major histocompatibility complex (MHC) molecules on antigen-presenting cells.
The specific area within the variable region that binds to the antigen-MHC complex is known as the complementarity-determining region (CDR), especially CDR3, which is the most variable and critical for antigen recognition.
In conclusion, it is the variable regions of the alpha and beta chains, shaped by genetic recombination and chain pairing, that enable T-cells to recognize and respond to an immense variety of antigens.