STOCKHOLM, Oct. 6—The 2025 Nobel Prize in Physiology or Medicine was awarded Monday to three scientists—Mary E. Brunkow, Fred Ramsdell, and Dr. Shimon Sakaguchi — for uncovering how the Immune System Discovery distinguishes between harmful germs and the body’s own cells.

The researchers’ work revealed a critical pathway known as peripheral immune tolerance, which prevents the immune system from mistakenly attacking healthy tissues. Their findings have helped explain autoimmune disorders such as Type 1 diabetes, lupus, and rheumatoid arthritis.

Announcing the award at the Karolinska Institute in Stockholm, Nobel Committee Chair Olle Kämpe said the discoveries were “decisive for our understanding of how the immune system functions and why we do not all develop serious autoimmune diseases.”

Brunkow, 64, is a senior program manager at the Institute for Systems Biology in Seattle. Ramsdell, 64, serves as a scientific adviser for Sonoma Biotherapeutics in San Francisco, while Sakaguchi, 74, is a distinguished professor at Osaka University’s Immunology Frontier Research Center in Japan.

How the discovery was made of Immune System Discovery?

In separate studies conducted in the 1990s and early 2000s, the scientists identified the role of specialized immune cells, now known as regulatory T cells, or T-regs. These cells act as the body’s internal control system, suppressing overactive immune responses that can lead to self-damage.

Sakaguchi’s experiments in mice showed that the thymus alone could not fully prevent immune overreactions. In 1995, he discovered the existence of regulatory T cells, which serve as a second line of defense against autoimmunity.

Six years later, Brunkow and Ramsdell, then working at a U.S. biotech firm, pinpointed a mutation in a gene called Foxp3 as the cause of autoimmune symptoms in lab mice. They later found that Foxp3 controls the development of regulatory T cells, linking the gene to human immune balance.

“From a DNA level, it was a really small alteration that caused this massive change to how the Immune system discovery works,” Brunkow said.

Sakaguchi later confirmed that Foxp3 determines the creation of regulatory T cells and ensures they can suppress overactive immune reactions. Together, these discoveries reshaped the understanding of immune regulation and opened new directions in immunology research.

Significance of the findings

According to Marie Wahren-Herlenius, a rheumatology professor at the Karolinska Institute, the research “opened a new field of immunology.” Before their work, scientists believed Immune System Discovery occurred mainly through the thymus. The trio’s discoveries revealed an additional, crucial layer of control.

Dr. Jonathan Schneck, an immunology expert at Johns Hopkins University, said the discovery has guided current studies seeking ways to boost regulatory T cells to treat autoimmune diseases more precisely. “Instead of suppressing the entire immune system, we can now aim to restore balance,” he said.

The American Association of Immunologists described the work as having “fundamentally reshaped our understanding of immune balance.”

While new therapies based on the findings are still being developed, experts believe the research will lead to treatments that minimize infections and side effects associated with traditional immune-suppressing drugs.

Reactions from the winners

Nobel Committee Secretary-General Thomas Perlmann said Sakaguchi was “incredibly grateful” when reached by phone early Monday. Later at a news conference, interrupted by a congratulatory call from Japan’s prime minister, Sakaguchi called the honor “a happy surprise.”

“There are many illnesses that need further research and treatment,” he said. “I hope these discoveries continue to lead to disease prevention.”

Brunkow said she initially ignored a call from Sweden, assuming it was spam. “When I told Mary she won, she said, ‘Don’t be ridiculous,’” her husband, Ross Colquhoun, recalled.

Ramsdell learned of the news while traveling. “My wife’s phone got service, and she said, ‘You won the Nobel Prize.’ I said, ‘No, I didn’t,’” Ramsdell said.

The 2025 Nobel Prize in Medicine is the first announced this year. The awards in physics, chemistry, literature, and peace will follow later this week, with the economics prize to be announced Oct. 13.

The three laureates will share 11 million Swedish kronor (nearly $1.2 million) and will receive their medals at a ceremony in Stockholm on Dec. 10, the anniversary of Alfred Nobel’s death.