<P> Gamma delta T cells (γδ T cells) represent a small subset of T cells that possess a distinct T cell receptor (TCR) on their surfaces . A majority of T cells have a TCR composed of two glycoprotein chains called α - and β - TCR chains . However, in γδ T cells, the TCR is made up of one γ - chain and one δ - chain . This group of T cells is much less common in humans and mice (about 2% of total T cells); and are found mostly in the gut mucosa, within a population of lymphocytes known as intraepithelial lymphocytes . In rabbits, sheep, and chickens, the number of γδ T cells can be as high as 60% of total T cells . The antigenic molecules that activate γδ T cells are still widely unknown . However, γδ T cells are not MHC - restricted and seem to be able to recognize whole proteins rather than requiring peptides to be presented by MHC molecules on APCs . Some murine γδ T cells recognize MHC class IB molecules, though . Human Vγ9 / Vδ2 T cells, which constitute the major γδ T cell population in peripheral blood, are unique in that they specifically and rapidly respond to a set of nonpeptidic phosphorylated isoprenoid precursors, collectively named phosphoantigens, which are produced by virtually all living cells . The most common phosphoantigens from animal and human cells (including cancer cells) are isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMPP). Many microbes produce the highly active compound hydroxy - DMAPP (HMB - PP) and corresponding mononucleotide conjugates, in addition to IPP and DMAPP . Plant cells produce both types of phosphoantigens . Drugs activating human Vγ9 / Vδ2 T cells comprise synthetic phosphoantigens and aminobisphosphonates, which upregulate endogenous IPP / DMAPP . </P> <P> All T cells originate from haematopoietic stem cells in the bone marrow . Haematopoietic progenitors (lymphoid progenitor cells) from haematopoietic stem cells populate the thymus and expand by cell division to generate a large population of immature thymocytes . The earliest thymocytes express neither CD4 nor CD8, and are therefore classed as double - negative (CD4 CD8) cells . As they progress through their development, they become double - positive thymocytes (CD4 CD8), and finally mature to single - positive (CD4 CD8 or CD4 CD8) thymocytes that are then released from the thymus to peripheral tissues . There is some evidence of double - positive T - cells in the periphery, though their prevalence and function is uncertain . In laboratory, T - cells can be converted into functional neurons within three weeks . </P> <P> About 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immunocompetent T cells . Increasing evidence indicates microRNAs, which are small noncoding regulatory RNAs, could impact the clonal selection process during thymic development . For example, miR - 181a was found to play a role in the positive selection of T lymphocytes . </P> <P> The thymus contributes fewer cells as a person ages . As the thymus shrinks by about 3% a year throughout middle age, a corresponding fall in the thymic production of naïve T cells occurs, leaving peripheral T cell expansion to play a greater role in protecting older subjects . </P>

Where does positive selection of t cells occur