UPDATE 1-Immune System Discoveries Win 2011 Medicine Nobel

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STOCKHOLM, Oct 3 (Reuters) - Three scientists who uncovered key secrets of how the body's immune system works have won the 2011 Nobel prize for medicine or physiology, the prize-awarding institute said on Monday.

Sweden's Karolinska Institute said in a statement that the prize went to U.S. scientist Bruce Beutler, Luxembourg-born Jules Hoffmann, based in France, and Canadian-born Ralph Steinman, based in the United States.

"This year's Nobel Laureates have revolutionised our understanding of the immune system by discovering key principles for its activation," the institute said.

Beutler and Hoffmann shared one half of the prize of 10 million Swedish crowns ($1.46 million) and Steinman won the other half.

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 so-called "therapeutic vaccines" that stimulate the immune system to attack tumours.

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

Medicine is traditionally the first of the Nobel prizes awarded each year. Prizes for achievements in science, literature and peace were first awarded in 1901 accordance with the will of dynamite inventor and businessman Alfred Nobel.

The award citation said scientists had long been researching the immune response by which man and other animals defend themselves against attack by bacteria and other microorganisms.

Beutler and Hoffmann discovered receptor proteins that can recognize microorganisms attacking the body and which activate "innate immunity", the first step in the body's immune response, the statement said.

"Ralph Steinman discovered the dendritic cells of the immune system and their unique capacity to activate and regulate adaptive immunity, the later stage of the immune response during which microorganisms are cleared from the body," it added. ($1 = 6.868 Swedish Crowns) (Reporting by Patrick Lannin and Anna Ringstrom in Stockholm and Ben Hirschler in London; Editing by Alastair Macdonald)