Failed kidney transplant. Close-up of a kidney being examined by a transplant surgeon after a failed renal transplant. Part of the ureter extends to lower left. The kidneys filter waste from the blood and excrete it as urine. Transplantation replaces a failed or failing kidney with a donor kidney. However, transplants can fail, usually due to rejection by the patient's immune system. The removal of a transplanted organ or tissue is known as explantation.