Nobel Medicine Prize Honors Discovery of Immune 'Brakes' That Prevent Self-Attack

The 2025 Nobel Prize in Physiology or Medicine was awarded to Mary E. Brunkow, Fred Ramsdell and Shimon Sakaguchi for their discoveries concerning peripheral immune tolerance, the mechanisms that stop the immune system from attacking the body's own tissues. The Nobel Assembly at the Karolinska Institutet in Stockholm announced the award on October 6, 2025, and the three laureates will share a prize of 11 million Swedish kronor.

A Hidden Layer of Immune Control

The immune system is designed to identify and destroy pathogens such as viruses and bacteria. Scientists have long understood that it also has to avoid targeting the body's own cells. Protection against those errors was for decades understood mainly as something that happens in the thymus, where developing immune cells are screened early in life. The laureates' work showed that a second, ongoing layer of control operates in the body's tissues after those cells have matured.

That second layer depends on a specialized group of immune cells known as regulatory T cells, which act as a brake on immune responses. Their existence was the subject of Sakaguchi's research, which identified these cells in the mid-1990s. Brunkow and Ramsdell pursued the genetic side of the question. Working with mice that developed severe autoimmune disease, they traced the problem to a mutation in a gene later named Foxp3. Their findings helped establish that the gene is central to the development and function of regulatory T cells.

From Mouse Mutation to Human Disease

The research carried a direct link to human health. Mutations in FOXP3 in people cause a rare, severe disorder known as IPEX syndrome, which leads to multiple autoimmune conditions, often beginning in infancy. The connection between the mouse studies and this human disease helped confirm that the same biological system operates in people, and it gave clinicians a clearer picture of what happens when immune self-restraint breaks down.

Why the Discovery Matters Now

Autoimmune diseases, in which the immune system attacks the body's own organs or tissues, include type 1 diabetes, rheumatoid arthritis, multiple sclerosis and inflammatory bowel disease. Many current treatments suppress the immune system broadly, which can leave patients vulnerable to infection. Understanding how regulatory T cells maintain balance offers a route toward more targeted approaches that would calm harmful responses while leaving the rest of immune defense intact.

The same principles are relevant in other areas of medicine. In cancer, tumors can exploit regulatory T cells to suppress the immune attack meant to destroy them. In organ transplantation, the goal is to prevent rejection without lifelong, broadly immunosuppressive drugs. Researchers have explored therapies built on regulatory T cells and related approaches in clinical studies, though most remain experimental and none should be regarded as a routine treatment at this stage.

A Field Built Over Decades

The prize recognizes work that unfolded over roughly three decades and that initially attracted limited attention, as researchers debated whether a distinct population of suppressive immune cells existed at all. The laureates' combined findings, spanning cell biology and genetics, helped settle that debate and opened a major research field. The award also reflects a broader pattern in which basic research on fundamental biological questions, pursued without an immediate clinical goal, later underpins new medicines.

For patients living with autoimmune conditions and for clinicians treating them, the recognition signals that the science behind immune self-tolerance is now central to medicine. Translating that understanding into safe, widely available therapies will take additional years of clinical testing, but the foundation the three scientists built is already shaping how researchers approach immune disease.