American scientists Mary Brunkow and Fred Ramsdell, together with Shimon Sakaguchi of Japan, won the 2025 Nobel Prize in Physiology or Medicine on Monday for discoveries explaining how the immune system spares healthy cells — paving the way for potential new treatments for autoimmune diseases and cancer.

Their discoveries shed light on peripheral immune tolerance, or “how we keep our immune system under control so we can fight all imaginable microbes and still avoid autoimmune disease,” said Marie Wahren-Herlenius, rheumatology professor at Sweden’s Karolinska Institute, which awards the prize.

The institute said all three laureates highlighted the role of regulatory T cells — white blood cells that act as “security guards” preventing immune cells from attacking the body’s own tissues.

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