The 2025 Nobel prize in physiology or medicine celebrates a discovery that answers one of medicine’s most profound questions: how does the immune system know when to attack, and when to stand down?
Most of the time, our defenses target dangerous infections and even cancers while leaving the body’s own tissues unharmed. But when that balance fails, the consequences can be devastating – from autoimmune diseases, where the immune system turns on healthy organs, to cancers, where it becomes too restrained to recognize and destroy tumor cells.
Three scientists – Mary E. Brunkow, Fred Ramsdell and Shimon Sakaguchi – uncovered how our bodies maintain this delicate control through a special class of immune cells called “regulatory T cells”. Their discovery revealed the immune system’s natural “brakes”: the internal mechanisms that prevent friendly fire but, in some cases, can also shield cancers from attack.
Understanding how these brakes work has already reshaped modern immunology. The same insight guiding new treatments for autoimmune diseases is now helping researchers fine tune cancer immunotherapies; adjusting the immune system’s restraint so it hits hard against tumors without turning against the body.
The immune system works like a highly trained Security force, patrolling every corner of the body to detect and destroy bacteria, viruses and rogue cells. But even the best security team can be dangerous without oversight.
Left unchecked, immune cells can mistakenly attack healthy tissue: the hallmark of autoimmune diseases such as type 1 Diabetes or Multiple Sclerosis. And when the system becomes too cautious, it can overlook genuine threats, giving cancers the chance to grow unnoticed.
For decades, scientists thought most of this immune “training” happened early in life, inside an organ called the thymus: a small gland above the heart where young immune cells learn which targets to attack and which to ignore. Those that fail this test are eliminated before they can cause harm.
But in the 1990s, Japanese immunologist Shimon Sakaguchi discovered there was more to the story. Through experiments on mice, he identified a previously unknown type of immune cell called a “regulatory T cell”: the peacekeepers of the immune system. These cells don’t attack pathogens themselves.
Instead, they hold the rest of the immune army in check, preventing unnecessary destruction. When Sakaguchi removed these cells in laboratory animals, their immune systems spiraled out of control, launching attacks on healthy organs. His work showed that these peacekeeping cells are essential for preventing the body from waging war on itself.
A few years later, Mary Brunkow and Fred Ramsdell found the genetic switch that makes these peacekeepers possible. They discovered that a single mutation in a gene called Foxp3 could leave both mice and human babies vulnerable to a rare but devastating autoimmune disorder called IPEX syndrome. The Foxp3 gene acts as the “on switch” for producing regulatory T cells. Without it, the immune system loses its referees and chaos follows.
The immune system relies on many types of T cells. T helper cells act as team captains, directing other immune cells to respond to infections. Much of my own research has focused on how these cells behave in HIV infection, where their loss leaves the immune system defenseless. Regulatory T cells belong to this same Family but serve the opposite role: they calm things down when the fight goes too far.
These peacekeepers keep the immune defenders focused on real threats rather than friendly targets. When they fail, autoimmune diseases emerge. But when they work too well, they can suppress immune attacks on Cancer, allowing tumours to hide and grow. Scientists are now learning how to fine-tune this balance: boosting the guards to control autoimmune disease, or easing the brakes so the body can fight back against cancer.
These discoveries have redefined how doctors think about immunity. Clinical trials are already testing therapies that expand regulatory T cells in people with Arthritis, diabetes or after an organ transplant; helping the body to tolerate its own tissues.
In cancer treatment, the opposite approach is used: blocking or disabling these peacekeepers to unleash a stronger immune attack on tumors. This is the principle behind modern immunotherapies, which have already transformed outcomes for patients with melanoma, lung cancer and lymphoma.
The work of Brunkow, Ramsdell and Sakaguchi shows how basic science can lead to profound changes in medicine. Their discoveries help explain not just why the immune system sometimes goes wrong, but how it can be guided back into balance – a balance that could one day prevent autoimmune diseases, improve transplant survival and make cancer therapies both safer and more effective.
The Nobel committee’s decision this year recognizes not only their scientific achievement, but also a vision of the immune system as something far more nuanced than an on-off switch. It’s a finely tuned orchestra and regulatory T cells are its conductors, ensuring the right notes are played at the right time, silencing those that might cause chaos.
By learning to adjust these biological “brakes” with precision, medicine is entering a new era. Treatments inspired by these discoveries are already improving lives and may, in time, transform how we prevent and treat disease across the spectrum, from autoimmunity to cancer.
Justin Stebbing is a Professor of Biomedical Sciences at Anglia Ruskin University. This article is republished from The Conversation under a Creative Commons license. Read the original article.
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