Your body's immune system protects you from potentially harmful substances, such as microorganisms, toxins, and cancer cells. These harmful substances have proteins called antigens on their surfaces. If your immune system identifies antigens that are foreign (not part of your body), it will attack the substance. In the same way, foreign blood or tissue can trigger a blood transfusion reaction or transplant rejection. To help prevent this, tissue is "typed" before the transplant procedure to identify the antigens it contains.
Though tissue typing ensures that the organ or tissue is as similar as possible to the tissues of the recipient, the match is usually not perfect. No two people (except identical twins) have identical tissue antigens. Immunosuppressive drugs are needed to prevent organ rejection. Otherwise, organ and tissue transplantation would almost always cause an immune response and result in destruction of the foreign tissue. There are some exceptions, however. Corneal transplants are rarely rejected because corneas have no blood supply -- immune cells and antibodies do not reach the cornea to cause rejection. In addition, transplants from one identical twin to another are almost never rejected.
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