Graft-versus-host disease (GVHD) is a complication that can occur after a bone marrow transplant in which the newly transplanted material attacks the transplant recipient's body.

See also: Transplant rejection

GVHD occurs in a bone marrow transplant involving a donor and a recipient. The bone marrow is the soft tissue inside bones that helps form blood cells, including white cells that are responsible for the immune response. Since only identical twins have identical tissue types, a donor's bone marrow is normally a close, but not perfect, match to the recipient's tissues. See: Histocompatibility antigen test

The differences between the donor's marrow and recipient's tissues often cause T cells (a type of white blood cells) from the donor's marrow to recognize the recipient's body tissues as foreign. When this happens, the newly transplanted bone marrow attacks the transplant recipient's body.

Acute GVHD starts within the first 3 months after transplant. Chronic GVHD starts more than 3 months after transplant, and can last for 3 years or longer.

Rates of GVHD vary from between 30 - 40% among related donors and recipients to 60 - 80% between unrelated donors and recipients. The greater the mismatch between donor and recipient, the greater the risk of GVHD. After a bone marrow transplant, the recipient usually takes drugs that suppress the immune system, which helps reduce the chances (or severity) of GVHD.