Rejection is a normal reaction of the body to a foreign object. When a new kidney is placed in a person's body, the body sees the transplanted organ as a threat and tries to attack it. The immune system makes antibodies to try to kill the new organ, not realizing that the transplanted kidney is beneficial. The anti-rejection medicine prevents your body from recognizing the kidney as a “foreign object.” Without enough of the medicine in your blood, your body “sees” the kidney and begins to attack it. Eventually you will damage enough of your kidney that you have to go back on dialysis.