Like a man on a flying trapeze, K.T. Jones has leapt from one medical study to another during his 15-year struggle with cancer, and he has no doubt that the experimental treatments he has received have saved his life.

Mr. Jones, 45, has an aggressive type of Hodgkin’s lymphoma that resists the usual therapies. At the start of his most recent clinical trial, his life expectancy was measured in months. That was more than three years ago. He received a drug that helped his immune system fight cancer — a type of immunotherapy, the hottest area in cancer research and treatment.

“I’ve been over 12 months now with no treatment at all,” he said. “I walk half-marathons.”

Mr. Jones is one of many patients who have benefited from lifesaving advances in immunotherapy. But he’s an outlier: He is African-American. As money pours into immunotherapy research and promising results multiply, patients getting the new treatments in studies have been overwhelmingly white. Minority participation in most clinical trials is low, often out of proportion with the groups’ numbers in the general population and their cancer rates. Many researchers acknowledge the imbalance, and say they are trying to correct it.

Two major studies of immunotherapy last year starkly illustrate the problem. The drug being tested was nivolumab, a type of checkpoint inhibitor, one of the most promising drug classes for cancer. In both studies, patients taking it lived significantly longer than those given chemotherapy.