Three Scientists Win 2025 Nobel Prize in Medicine for Breakthrough in Immune System Regulation

Stockholm, Sweden – October 6, 2025 — The 2025 Nobel Prize in Physiology or Medicine has been awarded to American scientists Mary E. Brunkow and Fred Ramsdell, along with Japanese immunologist Shimon Sakaguchi, for their groundbreaking discoveries that explain how the immune system prevents itself from attacking the body’s own cells.

The Nobel Assembly at the Karolinska Institute announced the award on Monday, recognizing the trio “for their fundamental discoveries relating to peripheral immune tolerance.” Their research uncovered the critical role of regulatory T cells, a class of immune cells that act as internal safeguards, maintaining balance in the immune system and preventing autoimmune diseases.

“Their discoveries have been decisive for our understanding of how the immune system functions and why we do not all develop serious autoimmune diseases,” said Professor Olle Kämpe, Chair of the Nobel Committee.

A Game-Changing Discovery

Sakaguchi, currently based at Osaka University in Japan, made the pivotal discovery in 1995 when he identified regulatory T cells while studying the thymus — the organ where immune T cells mature. His findings provided insight into why the immune system doesn’t routinely attack the body’s own tissues.

Building on his work, Brunkow and Ramsdell, working independently in the early 2000s, identified the Foxp3 gene — a crucial genetic factor responsible for the development and function of regulatory T cells. Their research linked mutations in this gene to IPEX syndrome, a rare but life-threatening autoimmune disorder.

“In the 1990s, finding a gene like Foxp3 was like searching for a needle in a haystack,” the Nobel Committee noted, emphasizing the extensive time and effort the researchers dedicated to their work.

In 2003, Sakaguchi connected the Foxp3 gene to his earlier discovery of regulatory T cells, conclusively demonstrating its central role in immune regulation.

Medical Impact and Global Praise

The laureates' findings have opened up new frontiers in the treatment of autoimmune diseases such as lupus, multiple sclerosis, and rheumatoid arthritis. Their work is also informing advancements in cancer immunotherapy and improving outcomes in organ and stem cell transplantation.

“This work has provided vital insights into the regulation of the immune system and is a powerful example of how basic physiological research can have a profound impact on human health,” said Professor Annette Dolphin, President of the UK’s Physiological Society.

Daniel Kastner, a senior investigator at the U.S. National Institutes of Health, added, “Regulatory T cells play an absolutely vital role in preventing or reducing the devastating impact of autoimmune diseases.”

Laureates React

Thomas Perlmann, Secretary of the Nobel Committee, said he spoke with Sakaguchi ahead of the official announcement and described him as “incredibly grateful.” Brunkow and Ramsdell, based in the United States, had not yet been reached due to the time difference.

Mary E. Brunkow currently works as a program manager at the Institute for Systems Biology in Seattle, while Fred Ramsdell is a co-founder of Sonoma Biotherapeutics, a biotech firm in San Francisco.

A Legacy of Scientific Achievement

This year’s award adds to a recent trend of Nobel Prizes in medicine recognizing major breakthroughs in human biology and disease prevention. In 2023, Katalin Karikó and Drew Weissman received the prize for their pioneering work on mRNA vaccines, which played a key role in the fight against COVID-19. In 2024, Victor Ambros and Gary Ruvkun were honored for their discovery of microRNA.

The Nobel Prize in Physiology or Medicine comes with a gold medal and a cash award of 11 million Swedish kronor (approximately $1 million USD).

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