Long-lived and residential intraepithelial lymphocytes are one of the largest populations of T-lymphocytes in the body. In contrast to other peripheral lymphocytes, IEL do not circulate in the blood stream or lymphatic system but reside in the epithelial layer in intestine. Such location in very special per itself – IELs patrol the condition within a single layer of cells and have dendritic look. They provide the first line of defense in case of pathogen crossing the epithelial barrier. IELs encompass surprising diversity of origins and divide into two main categories – conventional and nonconventional. It is based on molecular surface properties and the place where they acquire their final effector cytotoxic program.