An organ transplant is replacing an organ that is not working properly with a healthier organ. The most common organs to be transplanted are the heart, lung, liver, pancreas, kidney and intestines. In some cases, the body sees the transplanted organ as a threat and rejects the new organ. This can happen at any time after the transplant. Transplant patients often take immunosuppressant drugs to reduce the immune system's reaction and help prevent transplant rejection.