Nobel Prize in medicine goes to trio of scientists for discovering how the immune system is kept in check

2025 Nobel Prize in Medicine Awarded for Breakthrough Discoveries on Immune System Regulation

The 2025 Nobel Prize in Physiology or Medicine has been awarded to three scientists — Americans Mary E. Brunkow, Fred Ramsdell, and Japan’s Shimon Sakaguchi — for their pioneering work in uncovering how the immune system distinguishes between harmful invaders and the body’s own cells. The Nobel Committee announced the award on Monday in Stockholm, Sweden, recognizing their “fundamental discoveries relating to peripheral immune tolerance.”

Their research led to the identification of regulatory T cells, specialized immune cells that act as the body’s “security guards,” preventing immune attacks against its own tissues — a process that, when it fails, results in autoimmune diseases.

“Their discoveries have been decisive for our understanding of how the immune system functions and why most people do not develop severe autoimmune disorders,” said Olle Kämpe, chair of the Nobel Committee.

The findings have laid the groundwork for potential new treatments aimed at curing autoimmune diseases and improving therapies for cancer, organ transplants, and stem cell procedures. Diseases such as lupus, rheumatoid arthritis, and multiple sclerosis often have devastating impacts, but regulatory T cells play a crucial role in mitigating these effects, noted Daniel Kastner of the U.S. National Institutes of Health.

A Masterpiece of Evolution

The immune system, described by the committee as an “evolutionary masterpiece,” defends the body by identifying and attacking pathogens. However, some microbes camouflage themselves to mimic human cells, sometimes causing the immune system to mistakenly attack the body — a hallmark of autoimmune disease.

In 1995, Sakaguchi, an immunologist at Osaka University, made a groundbreaking discovery by identifying regulatory T cells while studying the role of the thymus in mice. His work explained why the immune system doesn’t constantly turn against the body.

Building on his research, Brunkow and Ramsdell — both American scientists — discovered in the early 2000s that mutations in a gene known as Foxp3 were responsible for severe autoimmune reactions in mice. They later showed that similar mutations in humans cause IPEX syndrome, a life-threatening autoimmune disorder.

Sakaguchi later connected their findings to his earlier work, demonstrating that the Foxp3 gene controls the development of regulatory T cells — solidifying the trio’s collective breakthrough.

“A Pleasant Surprise”

Speaking after the announcement, Sakaguchi said he was “pleasantly surprised” to learn of the honor and expressed hope that the discovery would “encourage further research into treatments for immunological diseases.”

Brunkow currently serves as a program manager at the Institute for Systems Biology in Seattle, while Ramsdell is co-founder of Sonoma Biotherapeutics, a biotechnology company based in San Francisco.

Annette Dolphin, president of the UK’s Physiological Society, praised the laureates’ work for providing “vital insights into immune regulation” and opening “new paths for treating autoimmune diseases, improving transplant outcomes, and developing advanced cancer therapies.”

She added, “This is a powerful example of how fundamental research in physiology can transform human health.”

The 2025 Nobel Prize in Physiology or Medicine carries a monetary award of 11 million Swedish kronor (about $1 million).

Last year’s award went to Victor Ambros and Gary Ruvkun for their discovery of microRNA, while the 2023 prize honored Katalin Karikó and Drew Weissman for developing the mRNA vaccine technology crucial to combating COVID-19.