In a landmark announcement from Stockholm on October 6, 2025, the Nobel Assembly at Sweden’s Karolinska Institutet named Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi as the joint winners of this year’s Nobel Prize in Physiology or Medicine. The trio, two Americans and one Japanese, will share the 11 million Swedish kroner prize—about $1.17 million—for their groundbreaking research on peripheral immune tolerance, a fundamental process that keeps the immune system from turning against the body it’s meant to defend.
Understanding Immune Tolerance
The immune system is a marvel, warding off thousands of viruses, bacteria, and other microbes every day. But its ability to distinguish between friend and foe is delicate. When this balance is lost, the immune system can attack the body itself, causing autoimmune diseases. The Nobel Committee praised the laureates for their “decisive” discoveries explaining why “we do not all develop serious autoimmune diseases,” according to committee chair Olle Kämpe.
Central to their research were T cells, the immune system’s front-line warriors. While most T cells are trained in the thymus to recognize and destroy invaders (a process known as central tolerance), Brunkow, Ramsdell, and Sakaguchi unraveled a second, crucial line of defense—peripheral immune tolerance. This process ensures that even if some rogue T cells escape, specialized “regulatory T cells” act as security guards, preventing these cells from attacking the body’s own tissues. Sakaguchi’s experiments in 1995 first identified these regulatory T cells in mice. Brunkow and Ramsdell followed, discovering in 2001 that gene mutations leading to problems in these cells can trigger autoimmune diseases. Sakaguchi then linked these findings together, showing how the body’s self-defense network is pieced together.
New Hope for Disease Treatment
The Nobel Committee and independent experts say these discoveries have already shaped new approaches to treating autoimmune disorders, and could impact therapies for cancer and organ transplantation. “Their research provided a ‘new handle’ on how to approach autoimmune disorders,” said Rickard Sandberg of the Nobel Committee.
The significance of this work isn’t lost on the laureates themselves. Sakaguchi, speaking to reporters outside his Japanese university lab, said, “I feel it is a tremendous honour.” The Nobel Prizes, established by Alfred Nobel’s 1895 will, continue to shine a spotlight on science that changes lives—and this year’s Medicine Prize is no exception.
