CHICAGO - The meeting between the 7-year-old boy and the 31-year-old man began awkwardly, as meetings of strangers often do. But then young Jacob Kowalik sized up the adult, and began to drop his guard.

They played tic-tac-toe. They talked about hockey. Jacob and his parents invited Marshal Davis to their house for pizza, Jacob's favorite meal.

There also was a gift, a husky dog stuffed animal, though it wasn't for young Jacob. It was FROM him, a small token of thanks to this man who'd given Jacob a gift that the boy was too young to fully understand.

Jacob's parents knew what Davis had done for their son, but how would they convey the magnitude of their gratitude?

"What do you say to someone who saves your child's life?" Jennifer Kowalik later asked.

So she kept it simple.

"Thank you," she said, reaching out to give Davis a hug.

It all started with a swab of the cheek more than a decade ago - nearly four years before Jacob was even born.

Then a young collegiate fencer at the University of Florida with his sights set on law school, Davis saw that a bone marrow registry called Gift of Life was having a drive on campus. He and some friends offered their DNA samples; it seemed like the thing to do. He filled out the required paperwork and gave it little more thought.

Then, in the summer of 2009, he received a phone call. A 5-year-old boy needed stem cells, and Davis was a match.

This was his chance to save a little boy's life.

"Wow," Davis thought. "How cool."

He was excited, but he was scared, too, as he considered the seriousness of the commitment he faced. There was medical risk to him, though generally manageable, he was told. Treatments before and during the cell harvest could be uncomfortable.

But if he didn't do it, the chances of finding an equally good match - and quickly - were slim.

For Jacob, this was the equivalent of winning the lottery. Doctors said the abnormalities in his marrow would eventually develop into acute leukemia without the transplant. Davis matched seven of eight of Jacob's characteristics that are considered when pairing donors and recipients; even Jacob's immediate family members were not nearly as good a match.

Davis knew he had to go through with it.

"We found a donor!" Becki Maloney, a nurse at Chicago's Children's Memorial Hospital, told Jacob's parents.

They, too, were thrilled but, like Davis, initially hesitant.

At this point, Jacob wasn't showing any external signs that he was sick. In fact, he appeared to be the picture of health - a gregarious, sports-loving first-grader with a big smile and a mischievous glint in his eye.

For him, a stem cell transplant came with its own risks and challenges, among them a common, potentially fatal complication called graft-versus-host disease.

But his parents also knew this had to be done. This donor, and his stem cells, represented Jacob's best chance at life.

In suburban Philadelphia where he lived with his wife Helen, Davis began treatments. As the stem cells multiplied in his blood stream, he felt achy, almost as if he had the flu - an expected symptom.

In October 2009, when it was time to harvest the stem cells, he traveled with his parents to the University of Maryland Medical Center in Baltimore. There, he was hooked up to a machine that, for six hours, drew his blood and sifted out his stem cells before returning that blood to his body.

The stem cells were put on ice and flown to Chicago, where Jacob and his family were waiting.

At this point, the process was still anonymous and would remain so for a year - though the Kowaliks were able to send cards to Davis, who had returned to normal life as an attorney and as fencing coach at Swarthmore College.

"Thank you for saving my brother," Jacob's older brother, Zach, said in a handmade card.

Finally, a year after the transplant, Davis and the Kowaliks were allowed to contact one another.

They were in touch by email at first and shared details about their lives. Jacob, Davis learned, had indeed developed graft-versus-host disease. He had lost his sandy brown hair and his skin had turned blotchy and red and is still sometimes so raw it bleeds. He also wears special glasses to protect his light-sensitive eyes.

Typically, Jacob is very strong, his parents say. Occasionally, the tears flow.

"He bottles it up," says his dad, Michael Kowalik, a salesman in the semiconductor business. "So it's a release. He needs to get it out."

Jacob also asks a lot of questions.

"Why did I get sick?" he sometimes asks his parents. "When will I get better?"

He is well aware that some of the other children he's met at the hospital haven't made it. But increasingly, his parents feel confident that he will.

His doctors do, too.

"It is very favorable for him now. The biggest concern is if he gets an infection," says Dr. Reggie Duerst, clinical director of the stem cell transplant unit at Children's Memorial.

Last month, Davis, his wife and his mother flew to Chicago for a national fencing championship and to meet Jacob and his family. Together, they visited the hospital where Jacob has been treated; a reporter and photographer were invited along.

Jacob was shy. But his parents thought it important to share their story so that others might register to be donors.

"By telling your story, you could save another kid's life," Jacob's mom told him, on the way from their home in Chicago's suburbs.

He stopped what he was doing. "What?" he asked.

"I think that's when he truly got it," his mom says. "The light bulb finally went off."

Jacob goes to the hospital twice a week for photopheresis treatments; his blood is exposed to light and medicine to help the donor cells adapt to his body. He's also on steroids, though he's being weaned from them.

During treatment, he played cards and joked around with Davis. Even after a single weekend together, there was a surprising comfort level there. The families were already planning their next visit together on the East Coast.

"So the beach next summer?" Davis asked Jacob. "I'll go, if you go."

"Maybe," Jacob said, grinning. "Maybe never."

He giggled. Davis shook his head and smiled, too.

Jacob now goes to school two days a week. He dreams of being back on the baseball diamond and soccer field next year.

Over and over again, the Kowaliks thanked Davis and his family, until his mother, Carol, had had enough: "No more thank-yous," she said.

"It felt good to know it was appreciated," her son added. "But this has been life-changing for us, too."