How did Nathan save Boyd's life?

Nathan saved Boyd's life by donating his stem cells. Boyd, who had been diagnosed with blood cancer, was in need of a stem cell transplant. Nathan, a young naval serviceman, had his cheeks swabbed and entered the stem cell registry while promising a day off from his commanding officer. Five years later, Nathan, who was deployed in Japan, was notified that he was a match for Boyd. The U.S. Navy facilitated Nathan's return for the stem cell donation, which ultimately saved Boyd’s life 1 2.

The two eventually met seven years later at a marathon event, creating a deeply emotional and impactful moment for both of them 3.