located in capable of mature T cell intraepithelial lymphocyte 2010-12-07T09:54:50Z MESH:D000075942 IEL Intraepithelial lymphocytes (IELs) are a large and diverse population of mainly T cells that reside within the epithelial cell layer of various mucosal tissues in the body, including the skin, intestine, and respiratory tract. These cells represent a substantial amount of the total lymphocyte pool and are most numerous in the intestine. They have distinct phenotypes and functional characteristics compared to lymphocytes residing in other tissues. Most intraepithelial lymphocytes express heterodimeric T-cell receptors (TCRs) consisting of αlpha and beta chains (TCRαβ), or less commonly gamma and delta chains (TCRγδ), together with the CD8 co-receptor (αβ or αα); there are also smaller subpopulations of IELs that areCD4+, or negative for both CD4 and CD8. Intraepithelial T cells typically express high levels of activation markers, tissue-homing integrins, and receptors and molecules characteristic of natural killer cells and cytotoxic T lymphocytes. TCRαβ+CD8+ IEL are able to recognize both transformed self antigens via natural killer receptors and foreign antigen via the TCR. The primary function of IELs is to maintain the integrity of mucosal barriers against pathogens. Given their strategic location within the epithelial layer, they are often the first immune cells that encounter invading infectious agents, such as bacteria, fungi, or viruses. Upon encountering pathogens, IELs activate and quickly respond by either directly killing the infected epithelial cells or alerting other immune cells. This rapid response, initiated by IELs, helps constrain the infection at the point of entry and is vital to maintaining the integrity of mucosal barriers. In addition to their central role in maintaining the integrity of epithelial barriers, IELs also play a crucial role in maintaining the physiological health of the epithelia by promoting repair and regeneration after injury. Furthermore, IELs are pivotal in supporting oral tolerance to dietary antigens and gut microbiota. An imbalance in the number and functionality of IELs is often associated with various human diseases including celiac disease, inflammatory bowel disease, metabolic syndrome, and colorectal cancer. (This extended description was generated by ChatGPT and reviewed by the CellGuide team, who added references, and by the CL editors, who approved it for inclusion in CL. It may contain information that applies only to some subtypes and species, and so should not be considered definitional.) A tissue-resident lymphocyte located within the epithelial layer of mucosal tissues, particularly within the gastrointestinal, respiratory, and reproductive tracts. Characterised by permanent residency, this cell typically expresses CD103 (integrin alpha-E), which binds to E-cadherin on epithelial cells, enabling epithelial retention in both mice and humans. Phenotypically, IEL displays an activated, antigen-experienced state characterised by the constitutive expression of cytotoxic molecules (granzyme B, perforin) and innate-like receptors (such as NKG2D), enabling rapid, localised surveillance and immediate response to epithelial stress. intraepithelial T-cell intraepithelial T cell https://cellxgene.cziscience.com/cellguide/CL_0002496 mucosal immune response epithelium