Lymphocytes: Gamma Delta

Antigen recognition by B and T lymphocytes is achieved by highly diverse heterodimeric receptors, the Band T-cell receptors (TCRs). T lymphocytes can be subdivided into two mutually exclusive subpopulations carrying TCR chains encoded by either a and b or g and d gene loci. These T-cell subsets are classically referred to as ‘ab’ and ‘gd’ T cells, respectively. Like other antigen receptors, the extensive structural diversity of gd TCR is generated through somatic recombination of gene segments termed variable (V), diversity (D) and joining (J). Moreover, in common with ab TCR, gd TCR is noncovalently linked to a transduction complex composed of several CD3 subunits, which triggers intracellular signalling cascades and subsequent activation of T-cell effector functions following antigen recognition. See also: B Lymphocytes: Receptors; Immunoglobulin Gene Rearrangements; T-cell Receptors gdT cells, which were fortuitously discovered in humans and rodents about a decade ago, are produced by all vertebrates studied thus far. While they generally represent a small fraction of T cells in primary and secondary lymphoid organs, they are greatly enriched in mucosal tissues, where they canmakeup the vastmajority ofT cells.Despite this, whether gd T cells fulfil functions distinct from, or redundant to, those achieved by ab T cells has long remained an enigma. Although this issue is not yet solved, several observations detailed hereafter suggest that gd T cells recognize a specific set of conserved antigens and play unique important roles in various immune responses and cell homeostatic processes. See also: Lymphoid System; Mucosal Lymphoid Tissues Distribution, Morphology and Phenotype