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. 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.