Proposed role of host γδ T cells in the induction of GVHD. Epithelia within the skin and intestinal tract produce “stress” proteins in response to the inflammation associated with total body irradiation that is recognized by resident γδ T cells. The γδ T cells activate host APC trafficking through the tissue and also secrete chemokines such as macrophage inflammatory protein-1 α (MIP1α), MIP1β, and RANTES (regulated on activation, normal T cells expressed and secreted). The activated host APCs then migrate to the regional lymph nodes and stimulate alloreactive αβ T cells, which are subsequently attracted to the site of epithelial injury, perhaps by γδ T-cell–derived chemokines, where they induce GVHD in the target organ.