Why might the immune system reject a transplanted organ?

A transplant organ is seen as foreign because the tissues of the donor organ don't match the tissues of the recipient at a cellular level. Such tissue matching depends on the major histocompatibility complex (MHC) molecules, the molecules that display a foreign antigen during an immune response. These molecules are quite distinct from person to person, providing a very personal chemical ID. So when the immune system of the recipient sees the molecular ID of the donor organ, it proceeds to unleash its destructive power on what it perceives as enemy tissue. The immune system's goal is total rejection of the organ.

Because the molecular ID of the MHC is genetically controlled, close relatives of the recipient are likely to have a closer tissue match than other people. (A transplant from one identical twin to another has no rejection problems.) And it's not unusual for people to step forward to donate a kidney to a relative when needed. But even with attempts at tissue matching, rejection of the transplant remains a problem.