gamma/delta T lymphocytes
T cells of the γδ subtype express on their surface a heterodimer involved in ligand recognition, the T-cell receptor (TCR), which is composed of a γ chain and δ chain distinct from, but related to, the α chain and β chain that form the TCR of αβ T cells.
In peripheral blood and lymphoid organs such as spleen and lymph nodes, γδ T cells are a small proportion of T cells. In contrast, they can constitute a large percentage of T cells within epithelia. For example, rodent skin contains exclusively γδ intraepithelial lymphocytes (γδ IELs) and, in most species, γδ IELs can account for as many as half of all T cells in the gut.
Activation of γδ IELs follows recognition by the TCR of ligands that remain ill-defined. Evidence that cell-surface receptors other than the TCR play a role in γδ IEL activation suggests that this process may be controlled through multiple types of receptor-ligand interactions.
The functional activities of γδ IELs can be grouped into the following general categories: cytolytic destruction of stressed or transformed cells; control of inflammation and developing immune responses; and modulation of epithelial cell growth.
Epithelial tissues house γδ T cells, which are important for the mucosal immune system and may be involved in controlling malignancies, infections and inflammation.
The antigenic molecules that activate γδ T cells are still largely unknown. However, γδ T cells are peculiar in that they do not seem to require antigen processing and MHC presentation of peptide epitopes although some recognize MHC class IB molecules. Furthermore, γδ T cells are believed to have a prominent role in recognition of lipid antigens.
There also exists a γδ T cell sub-population within the epidermal compartment of the skin. Named Dendritic Epidermal γδ T cells (DETC), these cells arise during fetal development and express an invariant and canonical Vγ3 Vδ1 T cell receptor (TCR).