Dr. Bryan Becker, president of the National Kidney Foundation and a transplant nephrologist, said the odds that people who are not related would have a "sibling-level" match like Jennifer Mazzotta-Perretti and her student Kevin O'Brien "are pretty low."

Once there's a matchin blood type, doctors look for matches in six human leukocyte antigens, which are small proteins on each cell that aid the function of the immune system. Donors and recipients don't have to match in all six categories, but the higher the number, the better the match.

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Because children receivethree antigens from each parent, siblings are often good matches.