Eli Roberts was just beginning to get tested as a possible kidney donor for Christine Tierney when a Brigham and Women’s social worker asked him a question he couldn’t answer: How might a transplant change his relationship with the recipient?

What relationship? he wondered. Roberts and Tierney scarcely knew each other. She was his friend’s boss. He was getting screened to be nice — but he also imagined a polite exit without going under the knife, figuring the odds were low that he would be a match.

That’s what Tierney assumed, too, short on hope last spring, eight years after being diagnosed with a life-threatening genetic condition that might shut down her kidneys entirely in another year. She remained deep on the waiting list for a deceased donor, and her husband and 15 friends had been ruled out as a match.

And she knew Roberts only as “the guitar guy,’’ the slender young man with the scraggly hair and colorful shirts who taught weekly lessons down the hall from the popular after-school program she runs in Cambridge.

They’d had just one exchange, sweet but surreal: when Roberts surprised Tierney by offering to get tested, and she gave him the number for the Brigham.

So neither could imagine the way their lives would become linked, or the friendship and admiration they would share by the end of the year.

“He’s like an amazing person, he really is,’’ Tierney said the other day. “What would Eli do?’’ is now her mantra for stressful situations. “Eli would definitely take the kind road, or he would make a joke.’’

Roberts, in turn, calls Tierney someone anyone would root for — “like she was deliberately designed to be charming, like her personality was written by a professional writer who knew what they were doing.’’

When Roberts first appeared in her doorway at the Cambridgeport School last April, Tierney wasn’t sure if she’d ever seen him do anything besides race down the hall, guitar and amp akimbo — perpetually scrambling, she would later learn, to get to a Friday night gig playing the Potbelly’s at Logan Airport, no time to spare.

He was a 24-year-old Berklee grad determined to cobble a living from teaching and playing music. He was also a practicing Catholic who tried to live his faith, organizing a mobile food pantry and carrying fresh socks in his backpack for the homeless people he was always engaging in conversation.

If Roberts was an extrovert, Tierney at 50 was shy around most adults, though she sprang to life among old friends and children. After stints in waitressing and hairdressing, she discovered herself as a writer, earning an MFA and becoming a Pushcart Prize-nominated poet. But that was her moonlight identity. A part-time job in her 20s had led to running the after-school program, known for affordable, high-quality care that mixes academics with lots of art and humor.

Which was why not just one but two parents had come forward as potential donors, though neither panned out. Then a niece materialized as a savior, passing even more tests, before getting cold feet.

That left Tierney with seemingly no options to counter polycystic kidney disease — only the grueling, stopgap prospect of thrice-weekly dialysis. 

As a Hail Mary, Tierney watched a webinar with her husband, Luis Blanco, about mounting a donor campaign, which emphasized publicity and positivity. She practiced talking about herself to strangers. A graphic-designer friend made a buoyant “Kidney for Christine’’ poster that told her story beneath a picture of Tierney getting attacked by a toy dinosaur.

Casey Long, an artist and storyteller who works with Tierney, wasn’t — as a cancer survivor — eligible to donate, but helped spread the word. When she told Roberts one night, he immediately said he would get tested — to the surprise of their friends.

Some wanted to make sure he realized kidney donation was actual surgery, not like donating clothing. “I don’t know, dude,’’ someone cautioned him. What if he needed that kidney himself someday?

But Roberts had already thought about it. He had contemplated what’s called altruistic kidney donation — giving anonymously, no specific recipient in mind — but hadn’t been ready, though he told himself he’d do it in an instant to save a friend. And here was a friend’s friend, the boss Long adored.

He went to the Brigham, where a test found he was not a match for Tierney. That was it, he figured — until they mentioned something called “paired donation,’’ in which multiple sets of donors and recipients exchange kidneys to facilitate a match, sometimes in chains involving dozens nationally.

“It seemed wrong to not do it if I could,’’ he said. “Suddenly it meant I could do something, which meant I had to.’’

So he consented to an array of additional blood tests, multiple 24-hour urine collections, an MRI, and other scans, plus psychiatric interviews, to make sure he was not just physically fit to donate but clear-eyed, proceeding without pressure or financial compensation.

But Roberts never quite told Tierney what he was up to, wondering if it was even OK to talk to her about it. So when the hospital donor center told her that he had agreed to pursue paired donation, she was stunned.

Roberts cleared one last test at the end of the summer, and then it was a matter of waiting — for algorithms to sort out the best sequence of matches, for hospitals and patients to finalize details, for everyone to be healthy and infection-free at once. It could be months. 

When school resumed, Tierney looked out for Roberts, filled with gratitude, admiration, disbelief. She had talked with her husband about what to do — invite him to dinner, write him a card, buy him a gift? — and decided she should wait for Roberts to approach. But that yielded just a couple of brief, flustered exchanges, neither sure what to say. “I just wanted to hug him,’’ Tierney said. 

Roberts felt a little flattered by making her tongue-tied, like a quarterback or homecoming king gliding down the hall. Then he felt guilty — he’d told himself he would pursue donation even if Tierney was a jerk, and she was the opposite of that, but now he feared he was squandering the moral value by relishing it.

Finally he slowed down for a moment one Friday before Thanksgiving, while racing to another gig at Logan. He told Tierney they could talk sometime — by phone, if she wanted. Or not.

“I want to be your friend,’’ he said, “but I have to go.’’

“I want to be your friend, too,’’ she said, grinning suddenly.

He raced toward the school exit, turning to shout back from the stairs: “Christine, I want to be your friend! Call me!’’

She beamed. And when she called, the conversation was easy. Soon they were texting regularly, and the awkwardness was gone.

In early December, they got word that an exchange had been arranged. Tierney would get a kidney from UCLA, and Roberts would send his to the Mayo Clinic in Minnesota. According to the National Kidney Registry, it was a five-donor, five-recipient swap.

Roberts went first, Dec. 12, his medical bills and expenses covered by Tierney’s insurance, a school-parent fund-raiser, and the hospital. They talked by phone right before.

Tierney’s operation followed the next morning, her new kidney arriving on a red-eye in a cooler with a satellite-tracking device, her surgery starting even before the kidney arrived.

Dr. Stefan Tullius, chief of transplant surgery, oversaw the procedures and called both a success.

An otherwise healthy patient like Tierney can get 20 or more years from a living-donor kidney, roughly double the amount provided by a kidney from a deceased donor, Tullius said. Of the 70 to 100 kidney transplants the Brigham performs annually, about half are from living donors; this was just the ninth in 2017 via paired exchange, the most yet.

Roberts, now 25, was out of the hospital in four days, and felt well enough to fly home to Virginia for the holidays the following week; last week he resumed teaching, though he is supposed to refrain from heavy lifting for three months.

“It’s really a medium-sized sacrifice,’’ Roberts said. “It’s not like joining the Army or becoming a parent.’’

At the Brigham, Tierney and Roberts texted regularly from their beds.

Blanco, Tierney’s husband, visited Roberts each day. They connected over their shared Berklee experience — Blanco had studied percussion there — and soon realized they had seen each other before, when Roberts was playing at a Potbelly downtown; Blanco was a rare customer who stopped to listen and leave a tip.

Tierney, home the following week, faces a three-month recovery. Though her immune system is not strong enough yet for crowds, she can now walk around the block.

She and Blanco have no idea how to begin to thank Roberts, welling up sometimes just thinking about it. “I just want to give him the stars and the moon,’’ she said.

For Christmas, they gave him a guitar-effects pedal. When Blanco delivered it, Roberts called it “the best gift I’ve ever received,’’ which is how Tierney described the kidney.

He said he’s been on a high, the happiness derived from a “sense of purpose’’ in helping others, elated by the daily texts about Tierney’s progress.

“It’s the freaking meaning of life. It feels great,’’ he said. “Even if Christine for some reason hated me, it would still feel great.’’

But she doesn’t, of course, which is why he caught himself at a New Year’s party in the middle of describing everything to a guy considering altruistic donation. Roberts heartily endorsed it, but he stopped when he “got to the part about Christine.’’

He considered “what a blessing her friendship is, what a blessing it is to know her.’’ He really couldn’t say what it might be like to give a kidney to a stranger.

Eric Moskowitz can be reached at eric.moskowitz@globe.com. Follow him on Twitter @GlobeMoskowitz.