The Nobel committee had been unaware of Ralph Steinman's death and it was unclear whether the prize would be rescinded because Nobel statutes don't allow posthumous awards.
Steinman, 68, who shared the prize with American Bruce Beutler and French scientist Jules Hoffmann, died on Sept. 30 of pancreatic cancer, according to Rockefeller University, which said he had been treated with immunotherapy based on his discovery of dendritic cells two decades earlier.
The cells help regulate adaptive immunity, an immune system response that purges invading microorganisms from the body.
The Nobel statutes don't allow posthumous awards unless a laureate dies after the announcement but before the Dec. 10 award ceremony. That happened in 1996 when economics winner William Vickrey died a few days after the announcement.
Nobel officials said they believed it was the first time that a laureate had died before the announcement without the committee's knowledge.
"I think you can safely say that this hasn't happened before," Nobel Foundation spokeswoman Annika Pontikis told the AP.
Nobel committee member Goran Hansson said the Nobel committee didn't know Steinman was dead when it chose him as a winner and was looking through its regulations.
"It is incredibly sad news," Hansson said. "We can only regret that he didn't have the chance to receive the news he had won the Nobel Prize. Our thoughts are now with his family."
The trio's discoveries have enabled the development of improved vaccines against infectious diseases. In the long term they could also yield better treatments of cancer, rheumatoid arthritis, type 1 diabetes, multiple sclerosis, and chronic inflammatory diseases, Hansson 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.
The discoveries have helped scientists understand why the immune system sometimes attacks its own tissues, paving the way for new ways to fight inflammatory diseases, Hansson said.
"They have made possible the development of new methods for preventing and treating disease, for instance with improved vaccines against infections and in attempts to stimulate the immune system to attack tumors," the committee said.
No vaccines are on the market yet, but Hansson told AP that vaccines against hepatitis are in the pipeline.
"Large clinical trials are being done today," he said.
Beutler, 53, is professor of genetics and immunology at The Scripps Research Institute in San Diego, 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, had been affiliated with Rockefeller University in New York since 1970, and headed its Center for Immunology and Immune Diseases.
"We are all so touched that our father's many years of hard work are being recognized with a Nobel Prize," Steinman's daughter, Alexis Steinman, said in the Rockefeller University statement. "He devoted his life to his work and his family, and he would be truly honored."
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 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.