Graft-versus-host-disease is a condition that can occur following bone marrow transplant. The donor's immune cells in the transplanted marrow make antibodies against the host's (transplant patient's) tissues and attack the patient's vital organs.

The condition may be acute or chronic, mild or severe. Severe cases can often be life-threatening. See also transplant rejection.

Bone marrow transplants are performed in specialized hospitals to treat and potentially cure certain types of leukemia or other cancers that have invaded the bone marrow. Doctors also perform experimental bone marrow transplants to treat non-malignant conditions, such as sickle cell anemia.

In the transplant, the host's bone marrow is destroyed with chemotherapy and/or radiation, and then the host receives replacement bone marrow from a donor. Most recipients undergo allogeneic transplants, which means they receive genetically matched bone marrow from a donor, usually a close family member or occasionally someone outside the family who has been found to be a match.

Sometimes, autologous transplants are performed. Here, the person is given back his or her own marrow once it has been purged of malignant cells.

While the goal of bone marrow transplantation is to eradicate tumor cells and thus the cancer, drugs are also given to suppress the host's immune system to permit the new donor marrow to implant without being destroyed by the host's immune system.

Graft-versus-host disease occurs when the new donor marrow makes antibodies against the host and tries to destroy the host as if the host were a disease or foreign material.

Varying degrees of graft-versus-host disease are viewed as an expected complication of bone marrow transplantation since tissue typing can find close, but not perfect, tissue matches between donor and recipient (see Histocompatibility antigens). Only identical twins have identical tissue types.

Some research suggests that a low level of GVHD may be helpful in fighting the disease that caused the need for the transplant in the first place.