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Intraepithelial lymphocytes are lymphocytes found in the epithelial layer of mammalian mucosal linings, such as the gastrointestinal tract and reproductive tract. Epithelium of small intestine contains approximately 1 IEL per 10 enterocytes. However, unlike other T cells, IELs do not need priming. Upon encountering antigens, they immediately release cytokines and cause killing of infected target cells. In the GI tract, they are components of gut-associated lymphoid tissue.
Based on expression of either an αβ T-cell receptor or a γδ TCR IEL T cells can be divided into two major groups. In mice both groups are retained in almost equal proportions. In humans, the majority of IELs are alpha beta T cells. 15% of IELs are gamma delta T cells and thus represent a minor component of human IELs. However, IELs significantly increase under certain conditions, such as celiac disease.