My friend Haym Hirsh (a computer science professor and a CMU graduate) writes:
Person A needs a new kidney. His relative B is willing to give one up for A, but B is not a good tissue match.
Person C needs a new kidney. His relative D is similarly willing to give one up for C, but D is not a good tissue match.
But what if B is a good match for C and D is a good match for A?