Following allogeneic stem cell transplantation, with an incomplete HLA match, donor lymphocytes proliferate and attack host tissues, resulting in graft versus host disease (GVHD). The organs most severely affected with acute GVHD, as in this case, are skin, liver, and GI tract. Thus, key findings that suggest acute GVHD include desquamative skin rash, jaundice, and diarrhea. Shown here are small intestinal villi with infiltration by leukocytes with focal necrosis. Most cases are mild. Chronic GVHD acts more like an autoimmune or immunodeficient state with a predominantly TH2 immune response. The major risk factor for chronic GVHD is a prior episode of acute GVHD. Fewer allogeneic stem cell transplant patients develop chronic GVHD, but it is much harder to treat.