GVHD is an immunologic disorder that results in severe gastrointestinal damage. It represents the response of immunocompetent donor cells to the histocompatibility antigens of the recipient. GVHD follows bone marrow or organ transplantation. Less commonly, it complicates maternal–fetal cell transfer in immunodeficient children (615) or transfusion of nonirradiated cells and blood products (616). GVHD also associates with malignant thymoma.

Basic requirements for GVHD reactions to occur include the following: (a) the graft must contain immunocompetent cells, (b) the host must be sufficiently genetically different from the graft to be perceived as antigenically foreign, and (c) the host must be unable to reject the graft (617). These conditions allow engrafted cells to react to the host through immunologically mediated processes. The incidence of GVHD ranges from <10% to >80%, depending on the degree of incompatibility, the number of T cells in the graft, patient age, and the nature of the immunosuppressive regimen (617). It should be remembered that GVHD may occur even in fully matched MHC donors and recipients due to incompatibilities in minor histocompatibility antigens. The incidence of GVHD is possibly higher in blacks than in other individuals.

CD8+, CD3+, and TiA1+ cytotoxic T cells mediate epithelial cell death in GVHD (619). CD8 cells recognize class II MHC-restricted antigens, producing the lymphokines that lead to the development of the enteropathy associated with
GVHD (620). Apoptosis may occur through the Fas/Fas ligand pathway (621).

Acute GVHD occurs within days (7 to 100) in recipients who are not HLA matched or in patients without any prophylaxis. Rarely, acute GVHD occurs later than 100 days posttransplant (618). It is characterized by epithelial cell death mainly in the gastrointestinal tract, liver, and skin, whereas chronic GVHD associates with fibrosis of these and other organs. Clinical features range from mild to intractable diarrhea, malabsorption, abdominal pain, protein-losing enteropathy, and severe malnutrition.