Intraepithelial lymphocytes of the intestinal mucosa: curiouser and curiouser.

Intraepithelial lymphocytes (IEL) of the intestine are discussed in light of recent findings concerning their ontogeny and T cell receptor expression. IEL are distinct from other peripheral T cells in many aspects, including their homing abilities and their phenotypic characteristics. IEL that express the gamma/delta T cell receptor are singular in that the thymus is not required for their maturation or for the selection of certain T cell receptor variable regions. Despite recent efforts, the in vivo function of IEL with gamma/delta T cell receptors remains unclear. Overall, IEL and the associated mucosa represent a complex lymphoid organ that will be the object of much future experimentation.