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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 on the floor 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. “You ready to go?”

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

The twins have now been discharged 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), a disease 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.

Weeks after his birth, Dominick was put on a transplant list and received stem cells from 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.

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

His parents asked him whether he wanted to donate. “We were straight up — ‘It’s going to be a big needle going into your back, bud,’ ” Pownall said. “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.

Pownall said her son told her he wanted to “help save my baby brothers.”

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. 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.”