Nobel Prize in Physiology or Medicine 2025: How the Body Keeps Its Immune System in Check

Every day, your immune system performs a remarkable balancing act. It must defend your body against invading microbes—bacteria, viruses, and parasites—without turning its powerful weapons against your own cells. But what keeps this system from going rogue?

That’s the question at the heart of this year’s Nobel Prize in Physiology or Medicine 2025, awarded to Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi. Their pioneering discoveries on peripheral immune tolerance have transformed our understanding of how the immune system stays under control—and opened new pathways to treat autoimmune diseases, cancer, and transplant complications.

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Understanding Immune Tolerance

The immune system’s job is simple in theory: identify and eliminate anything foreign. But in practice, the challenge is immense. Microbes can mimic the body’s own cells, and sometimes immune cells themselves make mistakes.

For decades, scientists believed that these mistakes were mostly prevented during immune cell development in an organ called the thymus—a process known as central tolerance. During this stage, T cells that react to the body’s own proteins are destroyed before they can circulate.

But this explanation wasn’t complete. Even with central tolerance in place, some potentially dangerous T cells still escape into the bloodstream. The immune system, it seemed, must have an additional safeguard.

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Shimon Sakaguchi and the Discovery of Regulatory T Cells

In the early 1980s, Shimon Sakaguchi, working in Nagoya, Japan, made a bold observation. He noticed that when newborn mice had their thymus removed, they didn’t just lose immune protection—they developed severe autoimmune diseases.

To investigate further, Sakaguchi injected healthy T cells from normal mice into those without a thymus. Surprisingly, this prevented disease. He concluded that some T cells acted as the immune system’s “security guards,” keeping other immune cells from attacking the body.

After years of painstaking research, Sakaguchi identified these peacekeeping cells in 1995. They were a new class of T cells characterized by the protein CD25, in addition to the familiar CD4 found on helper T cells.

He named them regulatory T cells (Tregs). These cells don’t destroy invaders directly—they control other immune cells, maintaining a balance between attack and tolerance.

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A Genetic Mystery from the 1940s

The next key to understanding immune regulation came from an unexpected source: a mouse mutation discovered decades earlier. In the 1940s, scientists in Tennessee noticed a strain of male mice with scaly skin, enlarged organs, and a fatal immune overreaction. The condition, called scurfy, puzzled researchers for years.

In the 1990s, molecular biologists Mary Brunkow and Fred Ramsdell, working at a biotech firm in Washington State, took interest in these mice. They suspected that the mysterious mutation could reveal why the immune system sometimes attacks the body—a clue that could help treat human autoimmune diseases.

Their challenge was enormous. The defective gene was somewhere on the mouse’s X chromosome, which contains around 170 million DNA base pairs. After years of mapping and analysis, they found the culprit: a previously unknown gene they named Foxp3.

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Linking Foxp3 to Autoimmune Disease in Humans

Brunkow and Ramsdell soon noticed similarities between the scurfy mice and boys suffering from a rare human disease called IPEX (Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked syndrome). These patients experienced severe autoimmune symptoms shortly after birth.

When they analyzed DNA samples from IPEX patients, the researchers discovered harmful mutations in the FOXP3 gene—the human counterpart of the scurfy gene. This finding, published in Nature Genetics in 2001, was a breakthrough moment. It revealed that FOXP3 mutations disrupt immune regulation in both mice and humans.

A few years later, Sakaguchi and others demonstrated that the FOXP3 gene is essential for the development of regulatory T cells. Without FOXP3, these vital cells cannot form, and the immune system spirals out of control.

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The Rise of Peripheral Immune Tolerance

Together, these discoveries unveiled the principle of peripheral immune tolerance—the mechanism that keeps mature immune cells from attacking the body’s own tissues. Regulatory T cells, guided by FOXP3, ensure that once an infection is cleared, the immune system calms down. They also prevent autoimmune attacks and regulate responses to transplanted organs.

This insight reshaped immunology. The concept that our immune system has internal “brakes” was once controversial but is now a cornerstone of medical research.

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From Discovery to Therapy

The impact of these findings goes far beyond theory. Understanding how regulatory T cells function has opened new frontiers in medicine.

In cancer research, scientists discovered that tumors often attract large numbers of regulatory T cells, which shield them from immune attack. Blocking or disabling these cells could help the immune system fight cancer more effectively—a strategy now being explored in clinical trials.

In contrast, for autoimmune diseases like type 1 diabetes, rheumatoid arthritis, and multiple sclerosis, researchers aim to boost regulatory T cell activity. Early studies suggest that treatments involving interleukin-2 (IL-2) can encourage Treg growth, helping calm overactive immune responses.

There are also experimental therapies where doctors isolate a patient’s own regulatory T cells, multiply them in the lab, and reintroduce them to restore balance. Some scientists even engineer these cells to target specific organs—such as a transplanted kidney or liver—reducing the risk of rejection.

These approaches, still under evaluation, hold immense promise for future personalized medicine.

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The Laureates and Their Legacy

Mary E. Brunkow earned her Ph.D. from Princeton University and now serves as Senior Program Manager at the Institute for Systems Biology in Seattle.

Fred Ramsdell, who completed his Ph.D. at UCLA, works as a Scientific Advisor at Sonoma Biotherapeutics in San Francisco.

Shimon Sakaguchi, a Distinguished Professor at Osaka University, continues to study immune regulation and its therapeutic potential.

Their combined work transformed how scientists view the immune system—from a simple defense force to a highly nuanced network of attack and restraint.

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Why This Nobel Matters

The 2025 Nobel Prize in Physiology or Medicine celebrates more than three decades of persistence and insight. It highlights how curiosity-driven research—spanning genetics, immunology, and molecular biology—can lead to discoveries that directly improve human health.

From understanding why mice got scaly skin to uncovering the genes that prevent our own bodies from self-destruction, Brunkow, Ramsdell, and Sakaguchi have given the world powerful tools to rethink disease and healing.

Their work reminds us that sometimes, the body’s greatest strength lies not in its ability to attack, but in knowing when to stop.

Source: Nobel Assembly at the Karolinska Institutet, “Press Release.”