American scientists Mary Brunkow, Fred Ramsdell and Japan's Shimon Sakaguchi won this year's Nobel Prize in Medicine for work shedding light on how the immune system spares healthy cells, creating openings for possible new autoimmune disease and cancer treatments.

The prize relates to 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, a rheumatology professor at the Karolinska Institute.

"Their 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," the prize-awarding body said in a statement.

The laureates identified so-called regulatory T cells, which act as the immune system's security guards that keep immune cells from attacking our own body, it added.

Sakaguchi told reporters outside his university laboratory that "I feel it is a tremendous honour," Kyodo news agency reported.

The winners for medicine are selected by the Nobel Assembly of Sweden's Karolinska Institute medical university and receive a prize sum of 11 million Swedish crowns (US$1.2 million), as well as a gold medal presented by Sweden's king.

Brunkow is senior programme manager at the Institute for Systems Biology in Seattle, while Ramsdell is scientific adviser at Sonoma Biotherapeutics in San Francisco. Sakaguchi is a professor at Osaka University in Japan.

After announcing the winners, the institute's Thomas Perlmann said specific therapies had yet to win market clearance but more than 200 trials on humans involving regulatory T cells are ongoing. (Reuters)