About a decade ago, when he was a student at the University of Florida, Marshal Davis made a spur-of-the-moment gesture during a campus drive for bone-marrow donors.

He got his cheek swabbed for a cell sample, signed some paperwork - and never gave it another thought.

Until his cellphone rang on a summer day in 2009.

The caller reminded Davis that he had registered as a potential donor years before. He barely remembered, but no matter. A 5-year-old boy in Chicago needed bone-marrow stem cells to fight off a disorder called myelodysplasia, often a precursor to leukemia.

Davis, the caller said, was a match.

By then a lawyer in Bucks County, he listened to the possible risks, did his own research, asked his doctor's advice, and talked to friends and family. Once again - though this time, not on a whim - he stepped up.

This spring, nearly two years after Davis' stem cells began coursing through Jacob Kowalik's body, the two finally met in Chicago to celebrate.

Jacob, now 7, still battles a common complication from the cell transplant. But the boy is better than he was. And in some ways, the same can be said for the donor.

"It was just an amazing opportunity to be a part of changing someone else's life," said Davis, 31, who lives in Buckingham Township and has a practice in nonprofit and corporate law in Warwick Township. "I never thought I'd be able to help somebody in that significant a way. To be able to do that, I can't really . . . put words to it."

Davis said he was contemplating running a local donor drive for the Gift of Life Bone Marrow Foundation, keeper of the registry. According to the website, giftoflife.org, it has helped facilitate more than 2,300 transplants.

Parents and siblings rarely are more than partial matches as donors of the all-important stem cells, which generate every type of mature blood cell, including infection-fighting white cells.

Jacob hit the jackpot with Davis, said Reggie Duerst, director of the Stem Cell Transplant Program at Children's Memorial Hospital in Chicago. "It was definitely a one-in-a-million situation," he said.

At first, Davis said, the prospect scared him.

Harvesting can be done in two ways. One involves withdrawing 2 percent to 3 percent of the donor's marrow by needle aspiration directly from the crest of the hip. In the other, less risky procedure - the method used with Davis - the cells are taken from the donor's blood.

After testing to confirm his health and double-check his suitability as a donor, Davis began taking a drug that would release stem cells from his bone marrow into his blood. The only unpleasant side effect was an achy feeling, similar to a flu symptom.

Next came an expenses-paid trip to the University of Maryland Medical Center, one of only a few hospitals in the United States that harvest stem cells. There, he was hooked up to an IV and the cells were filtered from his blood, a process that took about four hours.

Davis returned to Bucks County that night, feeling fine.

Meanwhile, the stem cells were on ice and being flown to Chicago, where Jacob was ready at Children's Memorial.

Until the end of the first year, neither recipient nor donor knew the other's identity, under privacy rules governing transplants. But Davis got periodic updates on the boy's progress - along with a card from Jacob's brother saying, "Thanks for saving my brother."

Jacob, however, had developed a common, and sometimes fatal, complication known as graft-versus-host disease. In essence, the donor cells were attacking the boy's body. He lost his hair. His skin became blotchy, and his eyes hypersensitive to light.

But increasingly, doctors are optimistic.

"He's doing amazingly well," Duerst said last week, while cautioning that Jacob has lingering skin problems and a higher risk of infection.

But in Mike Kowalik's eyes, his son's progress is clear. Jacob's hospital visits for photopheresis treatments - exposing his blood to light and medication to help his body adapt to the donor cells - are down from twice a week to once every other week.

When the Kowalik family learned that Davis, also a fencing coach at Swarthmore College, was headed to Chicago in April for a tournament, the trip became a momentous occasion.

Jacob was shy at first, but they ended up playing and giggling for hours. Jacob gave him a stuffed husky dog, and Davis brought souvenirs from the University of Florida and Swarthmore for the sports-loving youngster.

For his stem calls, Davis got a life lesson.

"You can't really have a bad day after that as long as you're healthy," he said, "and the people you love are healthy."

Contact staff writer Emilie Lounsberry at elounsberry@phillynews.com.