When Imani Cornelius was younger, and a blood disorder left her too tired to get up, much less go to school, she would lie in bed and think about two people.

There was the hero — maybe it would be a celebrity like Barack Obama — who would one day donate the bone marrow she needed for a stem cell transplant to fight her disease.

And in her darker moods, there was the villain — the person with similar genetics who had the chance years earlier to make the donation.

The volunteer who said no.

“I used to cry about it,” the Shakopee High School senior said. “That felt [at the time] like my one chance to be normal again. Why would they be so selfish? We never found out why they backed out.”

Some 6,000 people donate bone marrow and stem cells for transplant each year through Be the Match, a Minneapolis-based registry that connects donors to ailing recipients. But there is an equal number of people who sign up to donate but decline or don’t respond when actually asked to do so.

This 50-50 “commitment rate” has frustrated leaders of the National Marrow Donor Program, which operates the registry, and caused it to revamp the way it recruits donors as of next year. Among other things, the registry is adding a step to its recruiting drives at colleges — a top source of donors. Instead of signing up and giving genetic samples on site, students will provide their contact information and receive cheek swab kits in the mail. They also will receive profiles of patients awaiting matches.

“We need to show them the impact of not being available when you are called,” said Amy Alegi, marketing vice president for Be the Match.

Making the sign-up more cumbersome is a gamble for the nation’s primary bone marrow registry, which needs to recruit a large pool of donors due to the long odds of any one person being the exact genetic and blood type that a transplant patient needs. Even with 17 million people on the registry, some patients cannot find donor matches.

On the other hand, the organization is wasting time tracking down registrants who sign up but never really intend to donate bone marrow, Alegi said. The delay “can be detrimental to the health of the recipient. That can be devastating to the family.”

Reasons for saying no vary. Some can’t donate due to poor health or pregnancy when the moment comes. Others fear pain. Most donations are similar to giving plasma through an IV, but one in four involve surgery to extract marrow from the pelvic bone.

Black registrants donate only 27 percent of the time — a low rate that may reflect the mistrust that some minority groups have toward American health care. Trouble is, black people are more prone to sickle cell disease and other conditions that require transplants, so there is a greater need for black donors.

Some people sign up thinking it won’t matter, because only one in 430 registrants actually donate, said Dr. Linda Burns, the registry’s vice president and medical director. Most donors are called when they are in their 20s, which can be turbulent, she added. “They’re in that time of life and career when things can change relatively quickly.”

Slowly dying

Cornelius was born with Myelodysplastic Syndrome, a condition in which her own bone marrow wasn’t producing enough white blood cells — which primarily fight off infection. But the condition didn’t cause severe problems in early childhood.

Over time, though, the disease advanced and Cornelius suffered bad colds and extreme fatigue. By high school, the soccer player was no longer fit enough to run up and down the field. Many days, she skipped school and slept.

“People were like, ‘Is she still a student here?’ ” she said.

Fair or not, she rued the first registrant, the one who declined to donate and delayed her treatment.

Cornelius’ German and African-American ancestry is so unusual that she was featured in a National Geographic article on America’s blending ethnicity. But it also reduced her odds of finding a transplant match. It took years to secure a second match.

“I was slowly dying,” she said.

Tough to match

All human cells are marked with proteins called human leukocyte antigens (HLA), so that the body’s immune system recognizes them and doesn’t attack them.

When transplant recipients receive donors’ stem cells, they basically receive the donors’ immune systems as well. So without an HLA match, the donor immune system will attack the recipient’s body — a condition called graft vs. host disease that can be fatal.

Be the Match compares donors and transplant patients by eight HLA markers, and wants at least seven to be the same. Siblings have a one in four chance of matching. But for unrelated donors, the best odds of a match usually involve people with common racial or ethnic backgrounds.

“Some HLA types are incredibly common,” said Dr. Dennis Confer, the registry’s chief medical officer. “Up to 4 percent of the population of Caucasians have the same HLA type, and then it goes down from there to the point that some HLA types are incredibly rare.”

Experts have theorized that pandemics and plagues throughout history have depleted some HLA types and resulted in disparities.

The rising biracial population in the U.S. presents new challenges, because parents of different racial backgrounds are passing down new and unique genetic HLA combinations in their children.

“I don’t know Barack Obama’s HLA type, but I wouldn’t be surprised if he didn’t have any donors in our file,” Confer said, because the former president’s father was from Kenya and his mother was from Kansas.

Long wait ends

Cornelius’ wait ended in 2015, when a match was found in John Wiggins, a New Jersey jail guard. Wiggins, now 26, had been studying to enter the ministry when he signed up years earlier on the registry. The career path didn’t take, but the desire to donate remained.

“The initial call was that somebody is very sick and needs bone marrow. ‘Are you willing to go in for further testing?’ ” Wiggins recalled. “I was shocked that I was a match but I wanted to do it.”

Doctors wanted to use stem cells from Wiggins’ bone marrow, rather than his blood, because research has shown that this improves the odds of the transplant working — especially in children.

The extraction left him with bruising and hip pain for two days, Wiggins said. “Compare that to saving a life.”

Two years after the transplant, Cornelius has the energy to lead her high school robotics and rock-climbing clubs, play the saxophone, participate in lacrosse, and manage the school wrestling team. The 17-year-old also appeared in a documentary about bone marrow donation.

“It’s weird to say, but I’m a normal, functioning person again,” she said.

Be the Match has launched a Heritage Holds the Cure campaign to appeal to African-Americans — using testimonials from NBA star Mike Conley Jr. and urging people to understand the disease burden in minority communities.

Focus groups show that stories of patients waiting for transplants are persuasive. Contact information gained through the new online registration will allow Be the Match to share such stories with registrants on a personal level, which should increase their commitment, Burns said.

Potential donors also have been asked periodically since 2012 to recommit to the registry. People who don’t reply aren’t removed from the list, but those who do recommit tend to be reliable, she added.

Be the Match leaders also are discussing whether to offer registrants their own HLA information as a bonus, because it can be telling about their ethnic histories.

Donor meets hero

Medical privacy laws keep donors and recipients separated unless they agree through the registry that they want to correspond or meet.

Cornelius knew on the day of her transplant that she wanted to meet her donor. The size of the fluid bag containing the stem cells extracted from his bone marrow seemed huge.

“That’s a lot of bone marrow and blood to be giving,” she thought at the time, “and I don’t even know who this is.”

They waited a year, per registry policy, mostly to make sure Cornelius survived. Wiggins had donated blood before, but this was different. He was nervous until he learned that his recipient had come out of the transplant OK.

Their meeting occurred last month during the Be the Match annual convention.

In front of 1,500 people, Wiggins embraced Cornelius, who said thank you over and over. They cried. Cornelius’ mother hugged them both.

Cornelius discovered that her hero wasn’t a celebrity. In the end, that seemed right.

“It’s just some kind of regular person,” she thought. “That’s actually cooler.”