Such uncertainties regarding the diagnosis and treatment of GI GVHD stem from a lack of understanding of the pathophysiology of the disease. Cytokines produced by T lymphocytes, mononuclear phagocytes, and natural killer cells have been shown to play an integral role in the regulation of tissue damage. Human studies performed to date have examined peripheral blood cytokines, but results have been conflicting and with little clinical correlation. Current analysis of gut tissue has been limited mostly to animal subjects with little correlation to humans.