I've been nosing around online for information about kidney transplantation, and I was curious as to what would make a person a match, or exclude a person from donating. Here's what I have learned so far:
It is somewhat better for the patient to receive a kidney from a live donor vs. one who is recently deceased and a lot better for the patient to get a transplant without spending time on dialysis.
There are THREE factors that are used to match donors and recipients. One is blood type--same rules apply for receiving blood and organs (The Boy, being A positive, can receive O-, O+, A-, and A+ blood). The second is tissue type. With better anti-rejection medicines, this is less important, but we may as well try for the best we can get. There are six types of antigens in your tissue (three sets from each parent), and generally the best way to match the tissue is for the donor to be related to the recipient...but not necessarily. It's a more complicated version of people having different colored eyes, and The Boy could have the exact same color eyes--and the exact same tissue type--as someone he's never met before. The third, and probably most important, is called crossmatching. I'm assuming that this is related to the "type and cross" that is done whenever The Boy is about to receive a transfusion. White blood cells from the donor are mixed with blood from the potential recipient. If the white cells are attacked and die, the crossmatch is "positive" meaning that the donor cannot donate to that person. THIS is why parents should not donate blood to their kids if transplantation is ever going to be an issue--the child could develop an immunity to the parents' blood.
By the way, I have been getting much of this info from Living Donors Online. Also, I've been reading kidney.org for a little while.
They also have a list of factors that prevent a person from being a donor.