Gamma delta T cells represent a small fraction (1 - 5 %) of the overall T cell population but are enriched (more than 50 % of the T cell population) in epithelial cell-rich compartments like skin, the digestive tract, and reproductive organ mucosa. Gamma delta T cells are a subset of T cells defined by the genetic composition of their T Cell Receptor (TCR). All T cells are derived from common progenitor thymocytes, and while the majority of T cells express TCR chain heterodimers encoded by the alpha and beta gene loci, gamma delta T cells express TCR chains encoded by the gamma and delta gene loci. Subsets of gamma delta T cells are defined by the inclusion of invariant TCR V-(D)-J segments and are tissue- or context-specific. Gamma delta T cells have been described as a link between the adaptive and innate immune responses because, while they undergo V-(D)-J segment rearrangement to generate an adaptive, antigen-specific response, they can also be directly activated via the recognition of pathogen-associated or danger-associated molecular patterns, particularly natural phospho-antigens, by either the gamma delta TCR or other, non-TCR proteins, acting independently or together to activate gamma delta T cell effector functions. Like helper T cells, gamma delta T cells secrete particular effector cytokines in a subtype-and context-specific manner, however, unlike alpha beta T cells, most delta gamma T cells lack CD4 and CD8 and share a number of markers associated with natural killer cells or antigen-presenting cells such as Fc gamma RIII/CD16 and Toll-like receptors.