There I use the analogy of a kidney transplant (live not cadaveric), you can have a healthy kidney that you want to donate to a relative or dear friend, but you have to have a blood and tissue match. And then, they have to mix the two parties' blood and observe the reaction. If it's positive, the transplant cannot proceed, as they're incompatible. My mother used to say that she thought her former doctor was a damn good doctor, but he and I were completely incompatible personality wise.