All humans have a built-in defense system that allows the body to protect itself against disease. Unfortunately, this defense system cannot always distinguish between what is good and what is bad for us. This can cause the body to reject the transplanted kidney.
Generally speaking, the best matches come from close relatives since they are most likely to share genetic characteristics which reduce the chances for rejection.
Typically, a biological parent, brother, sister, or child of the candidate will make a better match. Other relatives, such as aunts, uncles, or cousins, may be an acceptable match. Non-relatives, such as a spouse or friend, may be a compatible match, too. It is important to note that even poorly matched kidneys from living donors have better 10-year graft survival rates than closely matched or mismatched deceased donor kidneys.