Nobel Prize in Physiology or Medicine Awarded for Immune System Research

Three scientists have just been awarded the 2025 Nobel Prize in Physiology or Medicine for a discovery that helps explain one of the immune system’s greatest balancing acts — how it attacks dangerous invaders while leaving our own bodies unharmed.

The prize was awarded to Mary E. Brunkow and Fred Ramsdell from the US and Shimon Sakaguchi from Japan for their breakthrough work on peripheral immune tolerance — in simple terms, how the body keeps its immune system from going rogue and causing 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 Olle Kämpe, chair of the Nobel Committee.

Meet the Body’s "Security Guards"

The immune system is like a hyper-vigilant security team, constantly scanning for threats. But sometimes, its random defense-building process accidentally creates immune cells that can attack the body itself.

Until the 1990s, scientists believed these dangerous cells were all dealt with in the thymus — where immune cells mature — through a process known as central tolerance. But Sakaguchi challenged that thinking. In 1995, he identified a new type of immune cell, now known as regulatory T cells, that roam the body and shut down any white blood cells that start attacking healthy tissue.

This was the first clue that the immune system has a second line of control outside the thymus — something called peripheral tolerance.

Then in 2001, Brunkow and Ramsdell independently discovered a key gene mutation that they called, Foxp3, whichcontrols the development of these regulatory T cells. They were studying a mouse strain prone to autoimmune diseases and found a mutation in Foxp3. When they looked at the human version of the gene, they found it was linked to a rare but serious autoimmune condition called IPEX.

In 2003, Sakaguchi confirmed that Foxp3 was the master switch for creating these protective immune cells. Together, their work revealed how the body polices its own immune system — preventing it from becoming a threat.

Why It Matters

This research has changed the way doctors and scientists understand — and treat — diseases where the immune system turns on the body. It’s already leading to new approaches for conditions like type 1 diabetes, multiple sclerosis, and rheumatoid arthritis.

And it goes both ways: In cancer, regulatory T cells can actually stop the immune system from fighting off tumors. So current treatments are trying to dial them down. Meanwhile, in autoimmune diseases and organ transplants, scientists are working on boosting these cells to reduce rejection and harmful inflammation.

"The discoveries have laid the foundation for a new field of research and spurred the development of new treatments," the Nobel committee said.

The three winners will split an 11 million Swedish kronor prize (about £870,000) and receive their medals from the King of Sweden at the official ceremony in December.