A small population of T cells also expresses markers that are found on NK cells, such as CD56; these are called NKT cells. The TCR a chains expressed by a subset of NKT cells have limited diversity, and in humans, these cells are characterized by a V region encoded by a rearranged Va24-Ja18 gene segment, with little or no junctional diversity, associated with one of three P chains. Because of this limited diversity, these cells are also called invariant NKT (iNKT) cells. Other NKT cells exist that have quite diverse antigen receptors. All NKT cell TCRs recognize lipids that are bound to class I MHC-like molecules called CD1 molecules. aP T cells that do not express NKT markers but recognize CD1-associated lipid antigens have also been described, and these cells may be CD4+, CD8+, or CD4-CD8- aP T cells. NKT cells and other lipid antigen-specific T cells are capable of rapidly producing cytokines such as IL-4 and IFN-y after activation, and they may help marginal zone B cells to produce antibodies against lipid antigens. NKT cells may mediate protective innate immune responses against some pathogens, such as mycobacteria (which have lipid-rich cell walls), and invariant NKT cells may even regulate adaptive immune responses primarily by secreting cytokines. However, the roles of these cells in protective immunity or disease in humans are unclear.
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