Three scientists won the Nobel Prize in medicine on Monday for discoveries about the immune system that opened new avenues for the treatment and prevention of infectious illnesses and cancer.

American Bruce Beutler and French scientist Jules Hoffmann shared the 10 million-kronor ($1.5 million) award with Canadian-born Ralph Steinman, the Nobel committee at Stockholm's Karolinska institute said.

Beutler and Hoffmann were cited for their discoveries in the 1990s of 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, known as innate immunity.

Steinman was honored for the discovery two decades earlier of dendritic cells, which help regulate adaptive immunity, the next stage of the immune system's response, when the invading microorganisms are purged from the body.

"Their work has opened up new avenues for the development of prevention and therapy against infections, cancer and inflammatory disease," the citation said.

Prize committee member Hans-Gustav Ljunggren told The Associated Press that drug companies are using the discoveries to develop better vaccines, though none are on the market yet.

"Some vaccines against hepatitis are coming out, large clinical trials are being done today," Ljunggren said.

In the long-term it will be possible to use the discoveries to improve the treatment of cancer, rheumatoid arthritis, type 1 diabetes, multiple sclerosis, and chronic inflammatory diseases, committee secretary Goran Hansson said.

Hansson said he had not been able to reach any of the winners before the announcement.

"Hoffmann for example is traveling in China and is difficult to reach," he said.

Beutler, born in 1957, is professor of genetics and immunology at The Scripps Research Institute in La Jolla, California. Hoffmann, 70, headed a research laboratory in Strasbourg, France, between 1974 and 2009 and served as president of the French National Academy of Sciences between 2007-2008.

Steinman, 68, has been affiliated with Rockefeller University in New York since 1970, and heads its Center for Immunology and Immune Diseases.

Hoffmann's discovery came in 1996 during research on how fruit flies fight infections. Two years later, Beutler's research on mice showed that fruit flies and mammals activate innate immunity in similar ways when attacked by germs.

Steinman's discovery dates back to 1973, when he found a new cell type, the dendritic cell, which has a unique capacity to activate so-called T-cells. Those cells have a key role in adaptive immunity, when antibodies and killer cells fight infections. They also develop a memory that helps the immune system mobilize its defenses next time it comes under a similar attack.

The trio's discoveries have enabled the development of new methods for treating and preventing diseases, including improved vaccines and in attempts to help the immune system to attack tumors, the committee said.

The medicine award kicked off a week of Nobel Prize announcements, and will be followed by the physics prize on Tuesday, chemistry on Wednesday, literature on Thursday and the Nobel Peace Prize on Friday. The winners of the economics award will be announced on Oct. 10.

The coveted prizes were established by wealthy Swedish industrialist Alfred Nobel _ the inventor of dynamite _ except for the economics award, which was created by Sweden's central bank in 1968 in Nobel's memory. The prizes are always handed out on Dec. 10, on the anniversary of Nobel's death in 1896.

Last year's medicine award went to British professor Robert Edwards for fertility research that led to the first test tube baby.

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Associated Press writer Malin Rising contributed to this report.