Transplant rejection is when a transplant recipient's immune
system attacks a transplanted organ or tissue. See also graft-
versus-host disease . Graft rejection; Tissue/organ rejection
Causes, incidence, and risk factors: 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 them. 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.