A graft rejection is an immune response by the body to destroy foreign cells in transplanted tissue. Graft rejections occur because the transplanted tissue or organ has antigens on its cells that do not match the person's own cell antigens. Only grafts from one identical twin to another are perfect matches, so most graft patients need to take immunosuppressive drugs to prevent their body from rejecting the graft.
Grafts are pieces of foreign tissue attached in some way to a patient's body. The graft can be from elsewhere on the patient's body, such as in the case of skin grafts. The graft may come from a cadaver, such as in a heart or a liver transplant, or from a living person, as is the case with many kidney and bone marrow transplants.
As mentioned, the best grafts in terms of the risk of rejection are those from one identical twin to another. Corneal transplants are unusual because they are rarely rejected. This is because corneas have no blood supply and, therefore, no immune system molecules reach the cornea.
Graft rejection occurs because the transplanted tissue has different antigens from the rest of the body. The patient's immune system is primed to attack foreign materials that display different antigens. Donors and recipients of grafts are checked for antigen compatibility before the transplant is performed to reduce the risk of graft rejection.
The most important of these antigens are the human leukocyte antigens (HLAs). These antigens are present on almost every cell in the body but get their name because white blood cells carry a lot of them. There are three main groups of HLA — HLA-A, HLA-B and HLA-DR. Each group contains many different antigens.
HLAs are inherited, so a person will get half of his HLAs from his mother and the other half from his father. Unrelated people tend to have very different HLA profiles, which is why relatives are the first tested for compatibility when a patient needs a transplant. Blood typing also has to be performed prior to a graft operation. Some of the blood groups O, A, B and AB are incompatible with each other, so a donor patient has to have an appropriate blood group as well as a suitable HLA profile.
A graft is rejected by the immune system if the antigen profiles don't match and the immunosuppressive drugs are not doing their job. The immunosuppressive drugs may be required for the rest of the tissue recipient's life to prevent graft rejection. Graft rejection is diagnosed if the graft is not working properly or if the patient is ill or feels unusual discomfort. Only rarely does a graft rejection manifest itself in fever, pain or swelling.