The effective TCR repertoire plays an essential role in the adaptive immune system. TCR, which locates on the T-cell surface, is responsible for interacting with peptide fragments of antigens that bound to major histocompatibility complex (MHC) molecules. In general, most of TCR consists of an α chain and a β chain as a heterodimer, and a minority of TCR is formed by a γ chain and a δ chain as an alternative. Commonly, for a healthy human, α/β TCR accounting for 95% and other 5% is γ/δ TCR. While the proportion between α/β TCR and γ/δ TCR is changeable due to ontogenesis, disease influence or different species. Similar to half of the Fab fragment of an antibody, each TCR chain is composed of a constant and a variable region which also has three complementarity determining regions (CDRs). Meanwhile, the TCR α chain and γ chain are generated by VJ recombination (just like the light chain of an antibody), and the β chain and δ chain undergo VDJ recombination (similar to the heavy chain of an antibody). During this rearrangement mechanism, the nucleotide additions or deletions at the junctions have endowed the TCR repertoire with essential diversity. In this way, the sequencing and analysis of TCR repertoire is a meaningful direction to enhance the understanding of the immune system, especially under different stimulation, such as infections, autoimmune diseases, and cancers, and contribute to discovering new therapeutic agents.
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