STOCKHOLM — Three scientists whose discoveries on the immune system opened up new avenues for prevention and treatment of infections, cancer and inflammations won the Nobel Prize in medicine.

American Bruce Beutler and French scientist Jules Hoffmann shared the $1.5 million award with Canadian-born Ralph Steinman, the Nobel committee at Stockholm Karolinska Institute said. Rockefeller University reports that Steinman died Friday. Beutler and Hoffmann were cited “for their discoveries concerning the activation of innate immunity.”

Steinman, 70, was honored for “his discovery of the dendritic cell and its role in adaptive immunity.”

“Their work has opened up new avenues for the development of prevention and therapy against infections, cancer and inflammatory disease,” the citation said. Beutler and Hoffman discovered receptor proteins that can recognize bacteria and other microorganisms as they enter the body, and activate the first line of defense in the immune system.

Steinman discovered dendritic cells in the immune system, which help regulate the next stage of the immune system’s response, when the invading microorganisms are purged from the body.

The discoveries enabled the development of new methods for treating and preventing diseases.