![]() The beta chain locus rearranges before the alpha chain and a functional beta chain has to be produced in order for the T cell to form a pre-T-cell receptor. The alpha and beta chains are encoded by different gene loci (alpha and beta TCR gene locus). TCRs are never secreted and always remain on the cell surface. A TCR has only one antigen-binding site, in contrast to the B-cell receptor, which has two. Each T cell has approximately 30,000 identical antigen receptors on its cell surface. A minority of T cells express other T-cell receptors made of different polypeptide chains, gamma and delta. The majority of T cells (approximately 90%) in the body express TCRs with alpha and beta chains. Each TCR exists as 2 different polypeptide chains (heterodimers) called the TCR alpha chain and TCR beta chain, and these are linked by disulfide bonds. T cells recognize these foreign antigens as peptides presented in the context of major histocompatibility complex (MHC) molecules through their TCRs. T cells, as part of the adaptive immune system, recognize foreign antigens when they are displayed on the surface of the body's own cells. During the process of rearrangement, DNA byproducts are generated called T-cell receptor excision circles (TREC) and these are used as markers of T cells that have recently emigrated from the thymus (TRECS / T-Cell Receptor Excision Circles Analysis, Blood). TCR gene rearrangement takes place in the thymus. The rearrangement of the T-cell receptor (TCR) through somatic recombination of V (variable), D (diversity), J (joining), and C (constant) regions is a defining event in the development and maturation of a T cell.
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