The 2025 Nobel Prize in Physiology or Medicine has been awarded for discoveries explaining how the immune system fights infections without harming the body's own cells.

The prize is shared by Japan's Shimon Sakaguchi and US researchers Mary Brunkow and Fred Ramsdell. Their research identified "security guards" within the immune system that eliminate potentially harmful immune cells before they can attack the body.

These discoveries are now being applied to develop new treatments for autoimmune diseases and cancer. The winners share a prize fund of 11 million Swedish kronor (£870,000).

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

How the Immune System Protects Us

Our immune system relies on white blood cells to detect signs of infection, including viruses and bacteria the body has never encountered. These cells use receptors formed in trillions of combinations, giving the immune system the ability to fight a wide range of invaders.

However, this randomness can produce white blood cells capable of attacking the body itself. Scientists previously knew that some of these dangerous cells were destroyed in the thymus, where white blood cells mature.

Discovery of Regulatory T-Cells

This year's Nobel Prize recognizes the discovery of regulatory T-cells, known as the immune system's "security guards." These cells travel throughout the body to neutralize any immune cells attacking the body's own tissues.

When this process fails, autoimmune diseases like type-1 diabetes, multiple sclerosis, and rheumatoid arthritis can develop.

The Nobel Committee noted, "The discoveries have laid the foundation for a new field of research and spurred the development of new treatments, for example for cancer and autoimmune diseases."

In cancer, regulatory T-cells can prevent the immune system from attacking tumors, so research is focused on reducing their activity. In autoimmune diseases, trials aim to boost regulatory T-cells to prevent the body from attacking itself. Similar strategies could also help reduce system transplant rejection.

Pioneering Experiments

Prof Shimon Sakaguchi from Osaka University performed experiments on mice without a thymus, causing them to develop autoimmune disease. He showed that injecting immune cells from other mice could prevent disease, suggesting a system exists to stop immune cells from attacking the body.

Mary Brunkow, from the Institute for Systems Biology in Seattle, and Fred Ramsdell, now at Sonoma Biotherapeutics in San Francisco, studied an inherited autoimmune disease in mice and humans, leading to the identification of a gene crucial for regulatory T-cell function.

Expert Perspective

Professor Annette Dolphin, president of the UK's Physiological Society, said, "Their pioneering work has revealed how the immune system is kept in check by regulatory T-cells, preventing it from mistakenly attacking the body's own tissues. This work is a striking example of how fundamental physiological research can have far-reaching implications for human health."