The 2025 Nobel Prize in Physiology or Medicine has been awarded to Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi for their discoveries on how the immune system distinguishes between harmful germs and the body’s own cells.
The announcement was made on Monday, October 6, 2025, by the Nobel Assembly at the Karolinska Institute in Stockholm, Sweden.
The three scientists were recognized for uncovering a key mechanism known as peripheral immune tolerance, which helps prevent the immune system from attacking healthy tissue. Their findings revealed the critical role of regulatory T cells, sometimes called T-regs, in keeping immune responses under control.
Professor Olle Kämpe, chair of the Nobel Committee, said the discoveries were “decisive for our understanding of how the immune system functions and why we do not all develop serious autoimmune diseases.”
Shimon Sakaguchi, aged 74, is a distinguished professor at the Immunology Frontier Research Center at Osaka University, Japan. In 1995, his experiments led to the discovery of regulatory T cells, a previously unknown type of immune cell that prevents the body from attacking itself.
In 2001, Mary E. Brunkow, 64, and Fred Ramsdell, also 64, working together in the United States, identified a mutation in the Foxp3 gene that disrupts these regulatory cells and causes autoimmune disease in mice. Their work confirmed how the gene controls immune balance and paved the way for understanding human autoimmune disorders such as type 1 diabetes, rheumatoid arthritis, and lupus.
Both Brunkow and Ramsdell are currently based in the United States. Brunkow serves as a senior program manager at the Institute for Systems Biology in Seattle, while Ramsdell is a scientific adviser at Sonoma Biotherapeutics in San Francisco.
Interestingly, the Nobel Committee faced brief delays reaching them due to the time difference with Stockholm, and Ramsdell was reportedly hiking “off the grid” when the award was announced.
Scientists worldwide have described the trio’s discoveries as transformative. The American Association of Immunologists said their work “has fundamentally reshaped our understanding of immune balance.”
Researchers are now using these findings to explore new treatments for autoimmune diseases, improve organ transplant outcomes, and strengthen cancer therapies. Experts say that boosting regulatory T cells could one day allow targeted treatment without the need to suppress the entire immune system.
Dr. Jonathan Schneck of Johns Hopkins University noted that although no new therapies have yet emerged directly from the research, it opened “an entirely new field of immunology” that continues to guide scientific progress.
The Medicine Prize is the first Nobel award announced in 2025, with Physics, Chemistry, Literature, and Peace Prizes to follow later in the week, and the Nobel Memorial Prize in Economic Sciences scheduled for October 13.
