Graft-versus-host disease is a common complication of certain types of tissue transplants, usually bone marrow or stem cells, but occasionally a blood transfusion. It only occurs when a patient is immunocompromised and the transplanted tissue is not an exact match.
Tissue rejection is often a complication of transplants. In graft-versus-host, the immune system recognizes the tissue is foreign and it is attacked by T-cells. However, due to the weakened immune system, the body is actually unable to destroy the transplanted tissue. As such, it continues to release T-cells and their efforts create cytokines. These build up in the blood and overwhelm the liver and other organs, resulting in the death of the patient.
There are two types of the disease - acute, which attacks soon after transplant and causes severe immediate symptoms, and chronic, which appears after more than 100 days and is progressive. The exact mechanism of each type is similar, but has important differences which must be accounted for in treatment.