T cell receptors
t cell receptors
T cell receptors (TR) are antigen receptors of the T cells of the adaptive immune response, acquired by jawed vertebrates (or gnathostomata) more than 450 million years ago and found in all extant jawed vertebrate species from fishes to humans. TR are anchored in the membrane of a T cell as part of the signaling T cell receptor (TcR = TR+CD3). TR alpha/beta recognize processed antigens, which are presented as peptides by the highly polymorphic major histocompatibility (MH, in humans HLA for human leukocyte antigens) proteins, whereas TR gamma/delta recognize nonpeptidic antigens. TR are made of two chains, alpha and beta, or gamma and delta, encoded by genes located in four major loci: the TR alpha (TRA) at 14q11.2, TR beta (TRB) at 7q34, TR gamma (TRG) at 7p14 and TR delta (TRD) at 14q11.2. Genes outside of these major loci are orphons. There are four TR gene types, variable (V), diversity (D) (for TRB and TRD), joining (J) and constant (C) genes which contribute to the TR chain synthesis. The variable domain at the N-terminal end of each TR chain results from a V-(D)-J rearrangement whereas the remaining of the chain, or constant region, is encoded by a C gene. The combinatorial and junctional diversity creates a huge potential diversity of 1012 specific TR per individual, the limiting factor being only the number of T cells that an organism is genetically programmed to produce.