Zac Oliver is only 4, but the British tyke has a worldwide following that is cheering his return to Children’s Hospital of Philadelphia for the final and most dangerous part of the T-cell therapy designed to save him from a ferocious form of leukemia.

Those supporters — ranging from school kids to celebrities — have donated more than $840,000 (650,000 British pounds) to a crowdfund created by Zac’s parents to cover his treatment and care. Contributions have poured in from Zac’s little hometown of Broseley, England, as well as Thailand, Russia, and many other places.

His mother, Hannah Oliver-Willets, posted on Facebook after she arrived with Zac on Tuesday: “After all the amazing support and hard work you did for Zaccy, he has finally made it to Philadelphia to have his T-cells put back in. Over the next few weeks, Zac will be having high-dose chemo, a bone marrow biopsy, and a lumbar puncture, all in preparation for his genetically modified T-cells to be infused. Fingers crossed everyone.”

Zac’s father, Mark Garbett, who will arrive in Philadelphia on Monday, said by phone, “It is quite an amazing story, to be fair.”

The story began in May, when Zac was diagnosed with an ultra-rare genetic subtype of acute lymphoblastic leukemia called near haploid. Continuous chemotherapy has not eradicated the disease, in which immature white blood cells grow out of control. A full-blown relapse would likely be fatal, and the conventional last-resort therapy — a stem-cell transplant — would offer little hope because his disease is so aggressive.

Confronted with the grim prognosis, Oliver-Willets, an occupational therapist, and Garbett, a heating contractor, began searching for a lifeline. (The couple separated before Zac’s birth, but Garbett is closely involved in his son’s life.)

They learned that England’s national health-care system — for which Oliver-Willets works — would soon begin covering Novartis' $475,000 T-cell therapy, Kymriah, which was pioneered at CHOP in collaboration with the University of Pennsylvania. But they feared that the system’s eligibility criteria would be a death sentence for Zac, as he would have to relapse after exhausting conventional therapies.

While Kymiah is approved in the United States for youngsters with resistant, relapsed leukemia, CHOP doctors have discretion to use it earlier in the disease course. They would not discuss Zac’s case, but the hospital said they have treated a small number of children with his rare subtype. Even in such unfavorable cases, Kymriah’s effectiveness has not diminished.

That effectiveness is what impressed Zac’s parents. In the global trial of 95 youngsters that led to U.S. approval, 80 percent went into remission, and more than 40 percent were alive a year later.

But that effectiveness comes with unprecedented costs and risks. The patient’s disease-fighting T cells must be siphoned from the blood, genetically programmed to recognize the cancer, multiplied for about three weeks, then reinfused in the child. The child undergoes an intense course of “conditioning” chemotherapy intended to make space in the bloodstream for the cancer-hunting T cells.

Zac’s T cells were removed when he was at CHOP in November. He is about to undergo the conditioning chemo.

Then he will face the scariest part of all. The T-cell reinfusion almost invariably triggers serious or even life-threatening complications, a result of immune overstimulation. The necessary hospital intensive care can inflate the cost of Kymriah treatment to more than $1 million.

“We know the risks," Garbett said. "But this is our only hope.”

CHOP has agreed to cover Zac’s therapy and care for $604,000, according to his parents, who are prepared to stay in Philadelphia through February.

In September, when they launched their crowdfunding campaign, the idea of raising such a huge sum was terrifying. How could they persuade strangers to donate to Zac amid a sea of equally desperate, deserving solicitations?

The secrets to their success were both mundane and extraordinary. They had a heart-wrenching story. The story was followed by social and traditional media in Britain and beyond. The extended family’s grassroots fundraisers — including a 24-hour football match and “Wear Red for Zac" day — caught on with hundreds of schools, businesses and organizations around the world. And Zac’s parents discovered they had influential friends-of-friends.

Music mogul Simon Cowell made a video and donated a big chunk of money. Dire Straits guitarist Mark Knopfler and professional footballer Jermain Defoe reportedly kicked in. A anonymous donor gave $130,000.

“We’ve been very lucky,” Garbett said.

With so much at stake, they’ve also tried to be financially scrupulous. The donations are administered, with legal restrictions, by a trust. After Zac’s needs are met, any leftover funds will go to research aimed at better understanding near haploid leukemia and other rare genetic subtypes.

“We wanted to be transparent with the trust,” Garbett said.