They were driving near the Maine Mall when they saw the message written in bright yellow paint on the rear window of another car: “Looking for someone to donate me their kidney. Must have type O blood.” There was a phone number.

“I looked over,” says Ashley Dall-Leighton, “and I go, ‘Oh, my God, that’s the saddest thing I’ve ever read.’ And he’s like, ‘What?’ And I read it out loud, and he said, ‘Did you get the number? Text her right now.'”

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And that’s how Josh Dall-Leighton, a 30-year-old corrections officer from Windham with three children, came to offer one of his kidneys to a woman he’d never met.

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Christine Royles of South Portland was diagnosed with lupus in December of 2013. A few months later, doctors discovered she had a second autoimmune disease, ANCA vasculitis.

They also told her that her kidneys were failing.

Only 23, she joined a waiting list of more than 100,000 people in the United States who need kidney transplants, according to the National Kidney Foundation.

Fewer than 17,000 receive one each year, with about a third of the kidneys coming from living donors. In 2013, 4,453 patients died while waiting for kidney transplants.

Christine didn’t like those odds. She decided to take matters into her own hands.

“I saw a story about this old man in a different state who was asking for a kidney for his wife,” says Christine, now 24. “He stood on the side of the road with a sign.”

Instead of standing on the side of the road, she decided to turn her car into a rolling billboard.

“My fiancé thought it was kind of weird,” she says.

But it worked.

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Kidneys are bean-shaped organs, each about the size of a fist, that sit in the rear of the abdominal cavity. Their job is to filter waste from the bloodstream. Humans are born with two, but it’s possible to lead a healthy life with one.

“You can live with one kidney,” says Jen Dalton, the clinical coordinator at Maine Nephrology Associates, “because that one kidney essentially takes over the job of two.”

People who need transplants are put on the national waiting list, but they can bypass it by finding a living donor themselves. Most living donors are family members, friends and acquaintances. But more and more, people are expanding the search.

“We see people using social medial or putting ads out for altruistic donors,” says Sean Roach, the public relations manager for the National Kidney Foundation. “It takes a lot of courage to put yourself out there like that.”

Finding a willing candidate is only the first step. A recent study presented to the American Society of Nephrology found that fewer than half of Americans meet the criteria to donate; those who aren’t eligible have health conditions such as obesity, hypertension and diabetes. Many others simply can’t afford to take the time off from work; the average donor stays in the hospital for at least two days and takes three to six weeks to recuperate.

A person who donates a kidney is generally able to lead a full, healthy life afterward but should be tested regularly for signs of potential kidney damage. They are advised to keep their weight and blood pressure down, but don’t require special diets or medication.

“In absolute numbers,” says Dr. John Vella, director of the Maine Transplant Program, “the chance of a kidney donor needing dialysis at some point is very much less than 1 percent.”

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Josh, who works as a corrections officer at the Southern Maine Re-Entry Center in Alfred, says he didn’t think twice after seeing the sign on Christine’s car.

“I just looked at my wife and said, ‘I have to try,'” he says. “I think it was the fact that I have three kids of my own, and that really resonated with me. If (my wife) needed a kidney and I couldn’t provide for her, I would hope that somebody else would kind of step up and help her out.” The Dall-Leightons’ twins, Mason and Christopher, are 10 months old. Their brother, Hayden, is 5.

Christine put Josh in touch with Maine Medical Center’s Transplant Program. After undergoing extensive tests this winter, Josh learned last month that he is a strong potential match.

He and Ashley texted Christine with the news.

“I started crying,” Christine says, “because, oh, my God, I can’t believe he’s gonna do this for me.”

Ashley was wary at first about the prospect of her husband donating a kidney, but quickly put her concerns aside.

“(Josh) just told me all along that this is what he needs to do, and if this is what he needs to do, then I support him 100 percent,” she says.

•••

Christine and the Dall-Leightons, who had been getting to know each other through texting and Facebook, finally met for the first time Tuesday.

Christine drove to Windham and knocked on the couple’s door, holding the hand of her 2-year-old son, Talan. Josh, holding 10-month-old Christopher in his arms, opened the door and gave her a hug.

She told him it’s hard to find the words to say thank you. But Josh said it isn’t necessary.

“I just want you to get better,” he says. “I just want to hear that donating helped you.”

For now, Christine continues to live a life consumed by her disease: weekly doctors’ appointments, trips to the hospital, mountains of medication. Every night, after she gets home from her job waitressing at Applebee’s and reads a bedtime story to Talan, she tethers herself for 10 hours to the portable dialysis machine that keeps her alive.

Because Christine has permanent kidney failure, Medicare is covering the cost of the transplant, which averaged roughly $262,000 in 2011, according to a U.S. Department of Health and Human Services website. Final testing on Josh should be completed in the next few weeks, and the transplant surgery is tentatively scheduled for mid-May. He expects to be out of work, on unpaid leave, for about a month.

Christine is now trying to raise money to help the family that’s helping to save her life.

“I’m shocked that someone is going to do this for me,” Christine says. “The fact that someone with a young family is going to take time off work to help some random person is unbelievable to me.”

But Josh says his gift to her isn’t a debt to be repaid. It’s simply the right thing to do.

As he talks, his twin sons squirm in his lap.

“I want these boys to know that if somebody needs help, you do whatever you can to help them,” he says. “I want them to know these aren’t just words I’m telling them. That I actually did something to help somebody.”