Organ transplantation is the operation in which an organ is removed from the donor body and then placed in a recipient body to replace a damaged or missing organ. The organs or tissues to be transplanted can be taken from living bodies or cadavers. Transplantation medicine is one of the most challenging and complex areas of modern medicine. The organs that can be successfully transplanted today include the heart, kidneys, liver, lungs, pancreas, intestine, thymus and uterus. The key to organ transplant are rejection problems such as the body's immune response to the transplanted organ, resulting in possible transplant failure and the need to remove the organ immediately from the recipient.