Many others are racing down the same path. Harnessing the immune system to fight cancer, long a medical dream, is becoming a reality. Remarkable stories of tumors melting away and terminal illnesses going into remissions that last years — backed by solid data — have led to an explosion of interest and billions of dollars of investments in the rapidly growing field of immunotherapy. Pharmaceutical companies, philanthropists and the federal government’s “cancer moonshot” program are pouring money into developing treatments. Medical conferences on the topic are packed.

All this has brought new optimism to cancer doctors — a sense that they have begun tapping into a force of nature, the medical equivalent of splitting the atom.

“This is a fundamental change in the way that we think about cancer therapy,” said Dr. Jedd Wolchok, chief of melanoma and immunotherapeutics services at Memorial Sloan Kettering.

Hundreds of clinical trials involving immunotherapy, alone or combined with other treatments, are underway for nearly every type of cancer. “People are asking, waiting, pleading to get into these trials,” said Dr. Arlene Siefker-Radtke, an oncologist at the University of Texas M.D. Anderson Cancer Center in Houston, who specializes in bladder cancer.