STOCKHOLM - Three scientists who unlocked secrets of the body's immune system, opening doors to new vaccines and cancer treatments, won the 2011 Nobel prize for medicine on Monday.

American Bruce Beutler and French biologist Jules Hoffmann, who studied the first stages of immune responses to attack, share the $1.5 million award with Canadian-born Ralph Steinman, who died last Friday. Steinman's discovery of dendritic cells in the 1970s is key to understanding the body's next line of defense against disease.

"He was diagnosed with pancreatic cancer four years ago, and his life was extended using a dendritic-cell based immunotherapy of his own design," the New York-based university said in a statement posted on its website.

"This year's Nobel laureates have revolutionized our understanding of the immune system by discovering key principles for its activation," the award panel at Sweden's Karolinska Institute said in a statement in Stockholm.

Lars Klareskog, who chairs the prize-giving Nobel Assembly, told Reuters: "I am very excited about what these discoveries mean. I think that we will have new, better vaccines against microbes and that is very much needed now with the increased resistance against antibiotics."

Beutler, 53, is based at the Scripps Research Institute in La Jolla, California. Luxembourg-born Hoffmann, 70, conducted much of his work in Strasbourg. They will share half the $1.46 million of prize-money. The rest goes to Steinman, 68, from Rockefeller University in New York.

The work of the three scientists has been pivotal to the development of improved types of vaccines against infectious diseases and novel approaches to fighting cancer. The research has helped lay the foundations for a new wave of "therapeutic vaccines" that stimulate the immune system to attack tumors.

Better understanding of the complexities of the immune system has also given clues for treating inflammatory diseases, such as rheumatoid arthritis, where the components of the self-defense system end up attacking the body's own tissues.

Beutler and Hoffmann discovered in the 1990s that receptor proteins act as a first line of defense, innate immunity, by recognizing bacteria and other microorganisms. Steinman's work, explained how, if required, dendritic cells in the next phase, adaptive immunity, kill off infections that break through.

Understanding dendritic cells led to the launch of the first therapeutic cancer vaccine last year, Dendreon's Provenge, which treats men with advanced prostate cancer.

"We live in a dangerous world. Pathogenic microorganisms threaten us continuously," the Nobel panel said, describing the work done over the decades in understanding our defenses.

"The first line of defense, innate immunity, can destroy invading microorganisms and trigger inflammation ... If microorganisms break through this defense line, adaptive immunity is called into action ... It produces antibodies and killer cells that destroy infected cells ... These two defense lines ... provide good protection against infections, but they also pose a risk ...: inflammatory disease may follow."

The award citation noted that the world's scientists had long been searching for the "gatekeepers" of immune response.

Hoffmann's pioneering research was conducted on fruit flies, highlighting how key elements of modern human biology have been conserved through evolution.

The immune system exists primarily to protect against infections but it can also protect against some cancers by targeting rogue cells before they proliferate.

Sometimes, however, the immune system goes into overdrive and attacks healthy tissue, leading to autoimmune inflammatory diseases, such as type 1 diabetes and multiple sclerosis, as well as rheumatoid arthritis. The effect is often compared to "friendly fire", when troops hit their own comrades in combat.

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