Three scientists won the Nobel Prize in medicine Monday for discoveries about how the immune system knows to attack germs and not our own bodies.

The work by Mary E. Brunkow, Fred Ramsdell and Dr. Shimon Sakaguchi uncovered a key pathway the body uses to keep the immune system in check, called peripheral immune tolerance. Experts called the findings critical to understanding autoimmune diseases such as Type 1 diabetes, rheumatoid arthritis and lupus.

Brunkow, 64, is now a senior program manager at the Institute for Systems Biology in Seattle. Ramsdell, 64, is a scientific adviser for San Francisco-based Sonoma Biotherapeutics. Sakaguchi, 74, is a distinguished professor at the Immunology Frontier Research Center at Osaka University in Japan.

The immune system has overlapping ways to detect and fight bacteria, viruses and other intruders. But sometimes certain immune cells run amok, mistakenly attacking people’s own cells and tissues to cause autoimmune diseases.

Scientists once thought the body regulated this system only in a centralized fashion. Key immune soldiers such as T cells get trained to spot bad actors and those that go awry in a way that might trigger autoimmunity get eliminated in the thymus.

The Nobel winners unraveled an additional way the body keeps the system in check if immune cells later get confused and mistake human cells for intruders, which is what happens when a person has an autoimmune disease.

Ramsdell was on vacation when the prize was announced, and did not find out until his wife's phone got service while they were driving.

"She said, 'you won the Nobel Prize.' And I said, 'no, I didn't.' And she said, 'yes, you did. I have 200 text messages that says you won a Nobel Prize' and I'm like, 'that's unbelievable," Ramsdell said.

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