CNN  —  When Allie Reimold boarded Flight 2223 in Houston a week ago, she didn’t expect to see him. It had been four years since they’d last visited in person. And eight years, almost exactly, since the budding scientist – on the darkest day of the year – had given the commercial airline pilot a gift that would link the two for life. Back then, United Airlines Capt. David Whitson had been facing a devastating diagnosis: acute myeloid leukemia. Healthy blood could bring the husband and father