A US-Japanese trio on Monday won the Nobel Prize in Medicine for research into how the immune system is kept in check by identifying its "security guards", the Nobel jury said.

The discoveries by Mary Brunkow and Fred Ramsdell of the United States and Japan's Shimon Sakaguchi have been decisive for understanding how the immune system functions and why we do not all develop serious autoimmune diseases.

Sakaguchi, a professor at Osaka University, told a press conference in Japan he hoped the award would "serve as an opportunity for this field to develop further... in a direction where it can be applied in actual bedside and clinical settings".

The Nobel committee was unable to reach the two US-based laureates to break the news to them in person.

"If you hear this, call me," the secretary

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