Jeff Swensen for The New York Times

PHILIPSBURG, Pennsylvania — Last spring Emma Whitehead, then 6, was near death from leukemia. She had relapsed twice after chemotherapy, and doctors had run out of options.

Desperate, her parents sought an experimental treatment at the Children’s Hospital of Philadelphia, one that had never before been tried in a child, or in anyone with the type of leukemia Emma had. It used a disabled form of the virus that causes AIDS to reprogram Emma’s immune system to kill cancer cells.

The treatment nearly killed her. But she emerged from it cancer-free, and about seven months later is still in complete remission. She is the first child and one of the first humans in whom new techniques have achieved a long-sought goal — giving a patient’s immune system the lasting ability to fight cancer.

Emma had been ill with acute lymphoblastic leukemia since 2010, when she was 5, said her parents, Kari and Tom. She is their only child.

She is among just a dozen patients with advanced leukemia to have received the experimental treatment, which was developed at the University of Pennsylvania. Similar approaches are also being tried at other centers, including the National Cancer Institute and Memorial Sloan-Kettering Cancer Center in New York.

"Our goal is to have a cure, but we can’t say that word,” said Dr. Carl June, who leads the research team at the University of Pennsylvania. He hopes the new treatment will eventually replace bone-marrow transplantation, an even more arduous, risky and expensive procedure that is now the last hope when other treatments fail in leukemia and related diseases.

Three adults with chronic leukemia treated at the University of Pennsylvania have also had complete remissions, with no signs of disease; two of them have been well for more than two years, said Dr. David Porter. Four adults improved but did not have full remissions. A child improved and then relapsed. In two adults, the treatment did not work at all.

Despite the mixed results, cancer experts not involved with the research say it has tremendous promise, because even in this early phase of testing it has worked in seemingly hopeless cases.

"I think this is a major breakthrough,” said Dr. Ivan Borrello, a cancer expert and associate professor of medicine at the Johns Hopkins University School of Medicine in Baltimore, Maryland.

Dr. John Wagner, the director of pediatric blood and marrow transplantation at the University of Minnesota, called the Pennsylvania results “phenomenal” and said they were “what we’ve all been working and hoping for but not seeing to this extent.”

A major drug company, Novartis, is betting on the Pennsylvania team and has committed $20 million to building a research center on the university’s campus to bring the treatment to market.

Researchers say reprogramming the patient’s immune system may also eventually be used against tumors like breast and prostate cancer.

To perform the treatment, doctors remove millions of the patient’s T-cells — a type of white blood cell — and, using a disabled form of H.I.V., insert new genes that enable the T-cells to kill cancer cells. The technique employs a disabled form of H.I.V. because it is very good at carrying genetic material into T-cells. The new genes program the T-cells to attack B-cells, a normal part of the immune system that turn malignant in leukemia. The T-cells home in on a protein called CD-19 that is found on the surface of most B-cells, whether they are healthy or malignant.

A sign that the treatment is working is that the patient becomes ill with raging fevers and chills — a reaction that oncologists call “shake and bake,” Dr. June said. Its medical name is cytokine-release syndrome, or cytokine storm, referring to the natural chemicals that pour out of cells in the immune system as they are being activated. The storm can also flood the lungs and cause perilous drops in blood pressure — effects that nearly killed Emma.

In patients with lasting remissions after the treatment, the altered T-cells persist in the bloodstream. Some patients have had the cells for years.

Dr. Michel Sadelain, who conducts similar studies at the Sloan-Kettering Institute, said:

"These T-cells are living drugs. With a pill, you take it, it’s eliminated from your body and you have to take it again.” But T-cells, he said, “could potentially be given only once, maybe only once or twice or three times.”

Researchers are not entirely sure why the treatment works, or why it sometimes fails. It is not clear whether a patient’s body needs the altered T-cells forever. The cells do have a drawback: they destroy healthy B-cells as well as cancerous ones, leaving patients vulnerable to certain types of infections.

So far, her parents say, Emma seems to have taken it all in stride. She went back to school this year with her second-grade classmates.

"It’s time for her to be a kid again and get her childhood back,” Mr. Whitehead said.