replacement of a diseased or damaged kidney with a healthy one obtained either from a living relative or a recently deceased person. Kidney transplant is a treatment for persons who have chronic renal failure requiring dialysis. Although kidney transplants were carried out in the late 1950s, clinically significant transplantation did not begin until around 1963, when the immunosuppressive drug azathioprine was developed to help counteract the rejection of the new organ by the body's immune system. Because a kidney from a related donor is less likely to be rejected by the body, transplants from living relatives are more successful than those from cadavers. Nevertheless, cadavers are today the most common source for transplants because of their greater availability and because they obviate the risk to living donors. The development of more effective immunosuppressive drugs such as cyclosporine has increased the success rates of both related donor and cadaver kidney transplants. Today, more than four-fifths of patients with transplanted kidneys will survive for more than five years.

Before transplantation, the immunologic characteristics of the recipient are carefully analyzed and a donor selected whose immunologic profile is matched as closely to the recipient's as possible. Traits used in determining a successful match include blood groups and tissue markers that enable the immune system to distinguish between the body's own tissues and foreign tissue. A transplant operation will be cancelled if the recipient has any infection, because of the risk that infection will spread, protected by immunosuppressive medication. Persons with chronic renal failure who also have active cancer are not considered candidates for kidney transplant, because immunosuppressive drugs may suppress the body's ability to contain the cancer.

The new kidney is implanted in the iliac fossa, a space in the groin area just below and to the side of the umbilicus; usually a right kidney is placed in the left fossa and vice versa to aid in making new attachments between blood vessels. The renal artery and vein are connected to the iliac artery and vein, and the ureter from the new kidney is either connected to the existing ureter or attached directly to the bladder. Formerly both of the recipient's kidneys were removed; they are now left in place unless they are infected or are too large to permit the new organ to be implanted.

Some degree of rejection, although treatable with medications, is fairly common, especially for cadaver kidneys. Some patients receive two or three kidneys before the body accepts one. Rejection may begin within minutes after the new organ is attached. Acute rejection, in which the tissues of the new kidney are injured by the immune system and the organ suddenly fails to function, can occur up to several years after operation but is most common in the first three months. Chronic rejection, in which deterioration of kidney function is more gradual, also may occur. Large doses of immunosuppressive drugs, along with drugs that retard the formation of blood clots, can halt acute rejection and save the transplant; if the medication does not help, the kidney is usually removed before infection or other complications set in.

Kidneys taken from living donors often begin to function immediately, while those from cadavers may take up to two weeks for tissues to adjust and become functional. If there are no complications from the transplant and no signs of rejection, the recipient can resume a virtually normal life within two months, although he must usually continue taking immunosuppressive drugs for several years. Because these drugs lower resistance to infection, however, other systemic complications may arise with time.