Then, Dr. June said, “The patient becomes a bioreactor” as the T-cells proliferate, pouring out chemicals called cytokines that cause fever, chills, fatigue and other flulike symptoms.

The treatment wiped out all of the patients’ B-cells, both healthy ones and leukemic ones, and will continue to do for as long as the new T-cells persist in the body, which could be forever (and ideally should be, to keep the leukemia at bay). The lack of B-cells means that the patients may be left vulnerable to infection, and they will need periodic infusions of a substance called intravenous immune globulin to protect them.

So far, the lack of B-cells has not caused problems for Mr. Ludwig. He receives the infusions every few months. He had been receiving them even before the experimental treatment because the leukemia had already knocked out his healthy B-cells.

One thing that is not clear is why Patient 1 and Patient 3 had complete remissions, and Patient 2 did not. The researchers said that when Patient 2 developed chills and fever, he was treated with steroids at another hospital, and the drugs may have halted the T-cells’ activity. But they cannot be sure. It may also be that his disease was too severe.

The researchers wrote an entire scientific article about Patient 3, which was published in The New England Journal of Medicine. Like the other patients, he also ran fevers and felt ill, but the reaction took longer to set in, and he also developed kidney and liver trouble — a sign of tumor lysis syndrome, a condition that occurs when large numbers of cancer cells die off and dump their contents, which can clog the kidneys. He was given drugs to prevent kidney damage. He had a complete remission.

What the journal article did not mention was that Patient 3 was almost not treated.

Because of his illness and some production problems, the researchers said, they could not produce anywhere near as many altered T-cells for him as they had for the other two patients — only 14 million (“a mouse dose,” Dr. Porter said), versus 1 billion for Mr. Ludwig and 580 million for Patient 2. After debate, they decided to treat him anyway.