On October 6, 2025, the Nobel Assembly at Sweden’s Karolinska Institute announced that the Nobel Prize in Physiology or Medicine would be awarded to three scientists—Mary Brunkow and Fred Ramsdell from the United States, and Shimon Sakaguchi from Japan—for their groundbreaking discoveries in immune system regulation. Their work, which centers on regulatory T cells, has opened the door to new approaches in treating autoimmune diseases, cancer, and a host of other conditions that have long challenged medical science.
The Nobel Prize in Physiology or Medicine, as tradition dictates, is the first announcement in the annual Nobel season. This year’s award recognizes what the Karolinska Institute describes as a fundamental breakthrough: the identification of a mechanism that prevents the immune system from turning against the body’s own tissues. According to the Washington Post, the laureates were honored “for fundamental discoveries about how the immune system keeps itself in check, preventing itself from attacking and destroying the body’s cells.”
At the heart of their discovery are regulatory T cells, a rare but powerful class of white blood cells that act as the immune system’s security guards. Their job? To keep the body’s own defenses from running amok. As Marie Wahren-Herlenius, a rheumatology professor at the Karolinska Institute, explained, “This year’s prize relates to how we keep our immune system under control so we can fight all imaginable microbes and still avoid autoimmune disease.”
Mary Brunkow, who currently serves as senior programme manager at the Institute for Systems Biology in Seattle, described the regulatory T cells as a “braking system that prevents the body’s immune system from tipping over into attacking itself.” In a moment that underscores the human side of scientific achievement, Brunkow learned of her Nobel win after being woken by her dog barking at a news photographer on her front porch. She reflected on the team’s work, saying, “They’re rare, but powerful, and they’re critical for sort of dampening an immune response.”
Fred Ramsdell, now a scientific adviser at Sonoma Biotherapeutics in San Francisco—a company he co-founded—was also recognized for his role in isolating the gene FOXP3. This gene serves as a marker for regulatory T cells and is essential for their development and function. The discovery of FOXP3 was pivotal, as it allowed scientists to identify and study these elusive cells, which had previously been difficult to distinguish from other immune cells.
Shimon Sakaguchi, a professor at Osaka University in Japan, was celebrated for his early work in identifying regulatory T cells and for helping to define their role in what’s known as peripheral immune tolerance. This process ensures that immune cells do not attack the body’s own tissues, a safeguard against autoimmune diseases such as Type 1 diabetes and rheumatoid arthritis. Speaking outside his laboratory, Sakaguchi told reporters, “I feel it is a tremendous honour,” as reported by Kyodo news agency.
The significance of this discovery cannot be overstated. According to The Wall Street Journal, the trio “uncovered a process that prevents the immune system from attacking our own tissues, called peripheral immune tolerance. The work unlocked a new field of research and potential therapies.” Their findings have laid the foundation for more than 200 ongoing clinical trials, testing therapies for autoimmune diseases, cancer, and even post-organ transplantation. Thomas Perlmann, secretary of the Nobel Committee, noted that while specific therapies have yet to win market clearance, the research has already spurred the development of new treatments for a wide range of conditions, from inflammatory bowel disease to various cancers.
The Nobel Prize itself is steeped in tradition and carries immense prestige. Winners receive 11 million Swedish crowns—about $1.2 million—and a gold medal presented by the King of Sweden. The ceremony is held in Stockholm on December 10, the anniversary of Alfred Nobel’s death, and is attended by members of Sweden’s royal family. The Nobel Prizes, established in 1901 through the will of Alfred Nobel, the Swedish inventor and industrialist, have recognized some of the most important contributions to science, literature, and peace. The economics prize, added later, is funded by Sweden’s central bank.
Past laureates in Physiology or Medicine include luminaries such as Alexander Fleming, who shared the 1945 award for the discovery of penicillin, and more recently, scientists whose work enabled the rapid development of COVID-19 vaccines. Last year, the prize went to Victor Ambros and Gary Ruvkun for their discovery of microRNA and its role in cellular specialization—a testament to the Nobel Committee’s focus on discoveries that fundamentally reshape our understanding of biology.
The path to this year’s Nobel Prize was paved by decades of research. Regulatory T cells, sometimes abbreviated as Tregs, had long been suspected to play a role in immune tolerance, but it wasn’t until Brunkow, Ramsdell, and Sakaguchi’s work that the field gained the tools and knowledge needed to study them in detail. By isolating the FOXP3 gene and demonstrating its crucial role, the scientists provided a molecular handle for researchers worldwide. This, in turn, led to a surge in clinical research, with more than 200 human trials now underway, exploring how Tregs might be harnessed to treat diseases that have, until now, been notoriously difficult to manage.
For patients with autoimmune conditions like Type 1 diabetes or rheumatoid arthritis, the promise of therapies that can recalibrate the immune system—rather than simply suppress it—offers a new sense of hope. Similarly, in the realm of cancer, where the immune system’s ability to distinguish friend from foe is often compromised, regulatory T cells could become a key part of future immunotherapies. And in organ transplantation, where the risk of rejection is a constant threat, manipulating Tregs may help ensure that transplanted tissues are accepted rather than attacked.
The Nobel Committee’s citation summed up the impact of the laureates’ work: “Their discoveries have laid the foundation for a new field of research and spurred the development of new treatments, for example for cancer and autoimmune diseases.” It’s a sentiment echoed by scientists around the world, who see this recognition as both a celebration of past achievements and a signal of the exciting possibilities ahead.
As the Nobel season continues—with prizes in physics, chemistry, literature, peace, and economics still to come—the story of Brunkow, Ramsdell, and Sakaguchi serves as a reminder of the power of curiosity-driven research. Their discoveries, rooted in the painstaking study of the immune system’s inner workings, have not only changed the way we think about disease, but have also given countless patients and families reason to hope for better treatments in the future.
The 2025 Nobel Prize in Physiology or Medicine stands as a testament to the enduring impact of fundamental science, and to the scientists whose curiosity and persistence continue to push the boundaries of what we know about ourselves.
