1) A reaction of donated stem cells against the patient's tissue.

2) A bodily condition that results when T cells from a usually allogeneic tissue or organ transplant and especially a bone marrow transplant react immunologically against the recipient's antigens attacking cells and tissues, that affects especially the skin, gastrointestinal tract, and liver with symptoms including skin rash, fever, diarrhea, liver dysfunction, abdominal pain, and anorexia, and that may be fatal -- abbreviation GVHD; called also graft-versus-host reaction.