GVHD is a common complication following a bone marrow transplant from another donor. GVHD occurs after transplant, when the donor's blood cells (called lymphocytes) recognize parts of your body, such as the skin, as foreign. A certain chemical, called Tumor Necrosis Factor, or TNF, also causes damage to the skin. The main effect on the skin is a red rash, when the skin GVHD is mild, but in more severe forms the skin can blister.