Following kidney (renal) transplantation, one possible complication is rejection of the new kidney. This occurs as a results of the body's immune system attacking (or rejecting) the newly transplanted kidney. After transplant, medicines known as "immunosuppressive" or "anti-rejection" drugs are given to transplant recipients to help prevent rejection of the transplanted kidney. If a transplant recipient stops taking these medicines, they almost always reject their transplanted kidney. However, in some exceptionally rare instances, transplant recipients who stop taking these drugs do not reject their kidney, and the kidney keeps working. The recipients are said at that point to "tolerate" the transplanted kidney, and this condition is referred to as "tolerance".