Definition of Transplant rejection:

Transplant rejection is a process in which a transplant recipient's immune system attacks the transplanted organ or tissue. See also: Graft-versus-host disease

Causes, incidence, and risk factors:

Your body's immune system usually protects you from substances that may be harmful, such as germs, poisons, and cancer cells.

These harmful substances have proteins called antigens on their surfaces. As soon as these antigens enter the body, the immune system recognizes them as foreign and attacks them.

In the same way, an organ that is not matched can trigger a blood transfusion reaction or transplant rejection. To help prevent this reaction, doctors "type" both the organ donor and the person who is receiving the organ. The more similar the antigens are between the donor and recipient, the less likely that the organ will be rejected.

Tissue typing ensures that the organ or tissue is as similar as possible to the tissues of the recipient, but the match is usually not perfect. No two people (except identical twins) have identical tissue antigens.

Doctors use a variety of drugs to suppress the immune system and prevent it from attacking the newly transplanted organ when the organ is not closely matched. If these drugs are not used, the body will almost always launch an immune response and destroy the foreign tissue.

There are some exceptions, however. Cornea transplants are rarely rejected because the cornea has no blood supply. Immune cells and antibodies are not able to reach the cornea to cause rejection. In addition, transplants from one identical twin to another are almost never rejected.

There are three types of rejection:

• Hyperacute rejection occurs a few minutes after the transplant, if the antigens are completely unmatched. The tissue must be removed right away so the recipient does not die. This type of rejection is seen when a recipient is given the wrong type of blood.
• Acute rejection may occur any time from the first week after the transplant to 3 months afterward. Everyone has some amount of acute rejection.
• Chronic rejection takes place over many years. The body's constant immune response against the new organ slowly damages the transplanted tissues or organ.