an incompatibility reaction (that may be fatal) in a subject (host) of low immunologic competence who has been the recipient of immunologically competent lymphoid tissue from a donor who is immunologically different from the recipient; the reaction, or disease, is the result of action of the transplanted cells against those host tissues that possess an antigen not found in the donor. Seen most commonly following bone marrow transplantation, acute disease is seen after 7-30 days and chronic disease weeks to months after transplantation, affecting, principally, the gastrointestinal tract, liver, and skin. SYN: GVH disease.