Transplant rejection occurs when the immune system detects the transplanted organ as foreign.
The immune system protects the body from substances that may be harmful. Antigens are found on the surface of these substances. If the immune system recognizes these antigens as foreign (not part of the body), it will attack them.
Transplant rejection works in the same way. A foreign tissue or organ can trigger a transplant rejection. Organ recipients need to take immunosuppressive drugs to prevent organ rejection.
WebMD Medical Reference from Healthwise