Three scientists who unlocked secrets of the body's immune system, opening doors to new vaccines and treatments for cancer, 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, shared the $1.5 million award with Canadian-born Ralph Steinman, who died of pancreatic cancer last week.

Steinman, 68, was working as an immunologist and cell biologist at Rockefeller University in the US.

"Steinman passed away on September 30," the university said in a statement. "He was diagnosed with pancreatic cancer four years ago, and his life was extended using a dendritic-cell based immunotherapy of his own design."

'This year's Nobel laureates have revolutionised 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.

'I also expect that there will be some development in the area of attacking cancers from the self-immune system. There are some promising things there.'

Annika Scheynius, a professor of clinical allergy research and a member of the panel, said: 'We are definitely sure that these discoveries will lead to health improvement ... They can improve the health of patients with cancer, inflammatory diseases, auto-immune diseases, asthma.'

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 10 million Swedish crowns ($1.46 million) of prize-money. The rest goes to Steinman, 68, from Rockefeller University in New York.

Self-defence

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 tumours.

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-defence system end up attacking the body's own tissues.

Medicine, or physiology, is usually 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 noted that the world's scientists had long been searching for the 'gatekeepers' of the immune response by which man and other animals defend themselves against attack by bacteria and other micro-organisms.

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

'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.

Hoffmann's pioneering research was actually 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. – Reuters