WASHINGTON—It was barely sunrise and Michael DeMasi Jr. was romping through the corridors at a children’s hospital in Philadelphia, balancing on a red line that was etched into the floor design and telling his mother to tie a balloon to him so he could “fly.”

The 4-year-old boy fancied himself a real-life superhero, wearing a blue T-shirt with photographs of his 4-month-old twin brothers, who were born with a rare immunodeficiency disease.

Michael’s little brothers — Santino, “Sonny,” and Giovanni, “Gio” — needed a bone-marrow transplant, and when his parents told him that he was a donor match, Michael told them that he wanted to save his brothers and would give them some of his.

“Hey, Michael,” his mother, Robin Pownall, called out as she filmed her young son early one Thursday morning in March, running ahead of her through Children’s Hospital of Philadelphia.

“Yeaaaah?” he responded, hanging back to let her catch up.

“You ready to go?” his mother asked.

“Yay!” he said, pumping his fists and jumping up and down.

“Where you going?” she asked him.

“To save my baby brothers,” he said.

“To save your baby brothers?” she repeated. “You’re the best.”

The preschooler knew what was about to happen, his mother said: a long needle would be pushed into the bone near his hip to extract the marrow, then the cells would be transplanted into his brothers.

He did it anyway.

And later in the day, when his brothers received the stem-cell infusion, Michael was there to watch, jumping and shouting, “That’s my bone marrow! That’s my bone marrow!” his mother said.

Nearly two months after the procedure, the twins were discharged last week from the hospital. Their mother posted a video on Facebook, showing Michael wearing a superhero T-shirt, smiling and standing between his little brothers.

“I saved you guys,” the preschooler said. “It’s time to go home.”

Santino and Giovanni were born in October with chronic granulomatous disease (CGD), in which the immune system does not work properly, meaning even a common cold could become life-threatening, their mother said. She and her fiancé, Michael DeMasi, were already familiar with the condition. Their oldest son, Dominick, 9, was also born with CGD and they knew it was a possibility that their other children would be born with the disease.

Following his diagnosis only weeks after his birth, Dominick was put on a transplant list and received stem cells from the umbilical cord of a donor. He is now considered cured, his mother said.

When the twins turned out to have CGD, Michael, who was not born with the condition, was tested and found to be a match. His parents talked to their doctors about the risks, such as pain and soreness and complications from anesthesia, then they sat down with Michael and tried to explain.

Michael knew that his little brothers were sick and he knew that they needed bone marrow. He also knew that he had been tested to determine whether he was a match, his mother said.

When his parents told him he was, he exclaimed, “I’m the match? I’m the match?” Pownall recalled.

His parents asked him whether he wanted to go through with the donation.

“We were straight up — ‘It’s going to be a big needle going into your back, bud,’ ” Pownall said she told him. “We asked him, ‘Do you want to do this? If you’re scared, you don’t have to.’ ”

Pownall said her son asked whether his bone marrow would allow his little brothers to come home.

She said yes.

He said OK.

But Pownall said the decision was not simple — she and her fiancé grappled with the option, wondering whether Michael understood what was being asked of him and whether it was right to ask him at all.

“Part of me was like, ‘Well, he’s 4. Maybe he doesn’t know what’s going on.’ But he did, and he was all for it,” she said.

Still, Pownall said, “We were in no way, shape or form going to push anything.” She said that had Michael hesitated or showed reluctance, she and his father would have put the twins on the bone marrow registry. But doctors told them that bone marrow from a matching family member was the best choice. “He was all for it and we had a good feeling about it. It’s amazing — he’s so proud. Such a brave little guy.”

She added that doctors at the hospital sat down with Michael and made sure he understood and that he was OK with it.

Art Caplan, a professor of bioethics and head of the division of medical ethics at the New York University School of Medicine, said situations in which children are the donors do present ethical questions, namely whether a child can truly understand the risks and grant their permission — a principle known as informed consent.

In addition, Caplan said, there may be risks to the donor — pain or other complications, and in instances in which the transplant is not successful, the donor may have to do it again or may begin to blame himself if the donation does not help or the recipient experiences a serious problem. He said that before such procedures are performed, he hopes to see an ethics committee review the circumstances to ensure there are no risks for the donor that the parents may not be considering.

But in general, “I’m inclined to allow it to happen,” he said, assuming that those involved understand and accept the risks.

It’s unclear whether the Children’s Hospital had an ethics committee review Michael’s case. A hospital spokesperson said she could not comment on Michael’s case, and when asked whether the hospital requires an ethics committee to weigh in on situations involving child donors, she said that she could not confirm that either.

On March 8, Michael marched down to the hospital’s patient check-in area.

His mother had told him that he was going to be “a real-life superhero,” and the small boy who loves Captain America as well as Spider-Man, Batman and Superman seemed excited to join their ranks.

The nurse told him that she was going to give him some “giggle juice” and he would go to sleep, his mother recalled.

Then Pownall was taken to a waiting room, where she said she sat anxiously, thinking about her young son. “I thought it was going to be harder on him than it was,” she said.

“He’s so small,” she added. “You don’t want your babies to be in pain.”

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Michael woke up with small welts on his back, but insisted he was not in pain, his mother said.

Pownall posted a video after his procedure, showing Michael with blue Popsicle juice all over his face. “I’ve got a blue tongue,” he said, sticking it out for the camera. “Because my Popsicle was blue.”

Hours later, the 4-year-old was jumping up and down, watching his bone marrow go to work.

The children’s aunt wrote on a GoFundMe page that Michael and his brothers were doing well.

“Michael did his procedure this morning for them to take the bone marrow cells,” the post read. “He is perfectly well, playing and all like nothing happened. Giovanni received the bone marrow cells around 2:30 p.m. & then Sonny received the bone marrow cells around 5:30 p.m. Both babies are doing well and sleeping. Now is time for their bodies to heal, with the new bone marrow cells to build up a healthy immune system.”

She posted an update last week, saying the transplants were successful and the twins were home.

Pownall said that now the twins, who are nearly 7 months old, show no signs of chronic granulomatous disease, though their doctors are monitoring them closely.

Michael, for his part, told the Post in a brief, distracted conversation that he felt “good” about helping his brothers.

The Post: “Hi Michael, my name is Lindsey and I’m a reporter.”

Michael: “WHAT?!”

Pownall: “She’s a reporter.”

Michael: “Oh.”

The Post: “Can you tell me what you did for your baby brothers?”

Michael: “I saved them because I’m a superhero!”

The Post: “How did you do that?”

(Michael moves away from the phone)

Pownall: “Michael, what did you give them?”

Michael: “Bone marrow!”

The Post: “Did it make them better?”

Michael: “Yes.”

The Post: “How do you feel about that?”

Michael: “Good.”

The Post: “Are you happy they’re home now?”

Michael: “Yes.”