A 4-year-old British boy battling leukemia has had a whirlwind visit to Children's Hospital of Philadelphia to begin what his parents hope will be life-saving immunotherapy.

Zac Oliver was diagnosed in May with an extremely aggressive subtype of acute lymphoblastic leukemia, in which immature white blood cells grow uncontrollably. British media have chronicled his parents' frantic campaign to crowd-fund more than $600,000 in barely three months to pay for treatment with Novartis' Kymriah, the revolutionary T-cell therapy pioneered at CHOP in collaboration with the University of Pennsylvania.

Zac and his mother, Hannah Oliver-Willets, arrived in Philadelphia on Nov. 13. His father, Mark Garbett, arrived on Wednesday, the day that Zac's T cells were siphoned from his blood at CHOP. The T cells — disease-fighting soldiers of the immune system — will be genetically programmed to recognize and attack his leukemia cells, multiplied over the next four to six weeks, then returned to his bloodstream. The re-infusion typically triggers serious side effects that require intensive care, adding to the $475,000 cost of Kymriah.

The family was scheduled to fly home to Broseley, a small town northwest of Birmingham, on Monday night, Garbett said during a phone interview early in the day.

"They've taken the T cells," Garbett said. "Zac is doing well. We're returning to England and we'll come back in December or January. We're just waiting for the hospital to tell us when to return."

Garbett, a heating contractor, separated from Oliver-Willets before Zac's birth but is closely involved in his son's life. Although Zac was terrified by the T-cell siphoning process, which required hooking him to a machine for five hours, he enjoyed a few outings in the city, including seeing a movie, Garbett said.

"Philadelphia is not like any city in England," Garbett said. "Everyone is so friendly. They have made us feel so welcome. People here are so kind."

In September, England's national health-care system announced a deal with Novartis to make Kymriah available to children who have relapsed and exhausted standard treatment options — chemotherapy, radiation, and stem-cell transplants. That's the same as the approved protocol in the United States.

But Zac's parents believe relapse would be a death sentence because of the vicious nature of his ultra-rare type of leukemia, called near haploid. Even though he has been on chemotherapy continuously since his May diagnosis, it has not eradicated the disease. CHOP agreed to treat him before relapse at a total cost of $604,000, according to his parents.