“It’s the nature of the beast,” said Martin Bachmann, a professor and immunologist at the Jenner Institute, which is affiliated with Oxford University. “I’m not sure you can get rid of the side effects — it’s really what you want.”

Chemotherapy, too, has side effects, but Dr. Kluger prefers immunotherapy’s trade-offs because the drugs may offer enduring control of cancer without the need for continued treatment. So she is joining others looking to address largely unanswered questions: Who is likely to be at risk, can the side effects be recognized before turning dangerous, and how should they be treated?

In June, Dr. Kluger and Dr. Herold submitted a grant proposal to the National Institutes of Health to study whether they could predict which patients would develop these symptoms. They based the proposal on a hypothesis that some patients have a biology or a genetic background that might make them more likely to have side effects. The proposal has not yet been funded.

Thus far, only a modicum of work has been done on these questions. Several studies found that older mice were more susceptible than younger mice to autoimmune reactions; another study, also in mice, found that obese subjects were more likely to have adverse effects.

“Old or fat mice were literally dead within hours,” said Dr. Murphy, the professor at Davis who believes too little is being done. He is well positioned to see the trends: In the past year, he sat on eight government grant review committees focused on immunotherapy, and he said only three out of 500 research proposals he reviewed focused on the toxicity side of immunotherapy.

Part of the problem, he said, is that the drug companies that are driving research prefer working with labs that support trials’ moving quickly. As a result, Dr. Murphy said, human trials are advancing faster than the background research can be done.

Hoping to push access to lifesaving drugs, the Food and Drug Administration has a “breakthrough therapy designation” that allows faster approval. Since 2012, the agency has granted breakthrough designation about 110 times, almost a quarter of them for immunotherapy.