That calls into question the prize for Ralph Steinman because Nobel rules forbid awarding the prize posthumously unless the winner dies after the award is announced, CNN reported.
In 1973, Steinman, a biologist at Rockefeller University, discovered dendritic cells, activators of the adaptive immune system, The New York Times reported.
He died Friday at 68 . He had been diagnosed with pancreatic cancer four years ago but extended his own life with dendritic cell-based immunotherapy he designed, the university said.
The Nobel Prize Web site states that since 1974, rules have specified a prize "cannot be awarded posthumously, unless death has occurred after the announcement."
In its announcement of winners Monday, the Nobel committee made no mention of Steinman's death, CNN said.
Later, the committee issued a statement expressing "deep sadness and regret."
"Our thoughts are with Ralph Steinman´s family and colleagues," the statement said.
Steinman, who was born in Montreal, received half the 2011 Nobel Prize in Physiology or Medicine, the Nobel committee announced. The other half went to Bruce Beutler, born in Chicago, and Jules Hoffmann, born in Echternach, Luxembourg, for their discoveries concerning the activation of innate immunity, the Nobel Assembly at Karolinska Institutet said in a release.
CNN reported that two people received Nobel Prizes posthumously before 1974 -- Dag Hammarskjold, winner of the Nobel Peace Prize in 1961, and Erik Axel Karlfeldt, who won for literature in 1931.