One of the winners of the Nobel Prize for Medicine - Ralph Steinman of Canada - had died of pancreatic cancer just days before.
The new laureates were Bruce Beutler of the United States, Luxembourg-born Frenchman Jules Hoffmann, and Ralph Steinman of Canada, who it was discovered Monday had died on September 30.
"This year's Nobel laureates have revolutionised our understanding of the immune system by discovering key principles for its activation," the jury said in a statement.
The three were lauded for their work on the body's complex defence system in which signalling molecules unleash antibodies and killer cells to respond to invading microbes.
Understanding this throws open the door to new drugs and also tackling immune disorders, such as asthma, rheumatoid arthritis and Crohn's disease, in which the body mysteriously attacks itself.
"Their work has opened up new avenues for the development of prevention and therapy against infections, cancer and inflammatory diseases," the jury said.
The Nobel jury was caught off guard though when it discovered hours after announcing the prize that Steinman had died of pancreatic cancer on Friday the age of 68.
Prize regulations stipulate the award cannot be given posthumously.
However the Nobel Assembly at the Karolinska Institute that awards the prize held a meeting late Monday to discuss how to handle the situation, deciding ultimately that Steinman would remain a Nobel laureate given the "unique" situation.