Allison's work explored how a protein can function as a brake on the immune system, and how the immune cells can combat tumors if the brake is released.
Researchers working under the direction of Allison and Honjo discovered methods of removing constraints on "T cells" that fight invaders by modulating immune responses.
The scientists' work in the 1990s has since swiftly led to new and dramatically improved therapies for cancers such as melanoma and lung cancer, which had previously been extremely hard to treat.
Allison and Honjo discovered two different "brake" proteins that act slightly differently.
The seminal discoveries represent a "paradigmatic shift in the fight against cancer", the Nobel committee said, since they do so not by targeting the tumor cells but by tweaking the immune system.
Two scientists have been jointly awarded the Nobel Prize for medicine for "landmark" research into how the body's natural defences can fight cancer. "I didn't set out to study cancer, but to understand the biology of T cells, these incredible cells [that] travel our bodies and work to protect us".
Medicine is the first of the Nobel Prizes awarded each year.
Thanks to Allison's doggedness, anti-CTLA-4 therapy is now an accepted therapy for cancer and it opened the floodgates for a slew of new immunotherapies, Krummel said.More news: Graham rips Dems for turning Kavanaugh hearing into 'unethical sham'
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Research by Allison's team has meanwhile led to the development of a monoclonal antibody drug, which was approved by the US Food and Drug Administration (FDA) in 2011 for the treatment of melanoma. From 1977-1984 he was a faculty member at University of Texas System Cancer Center, Smithville, Texas; from 1985-2004 at University of California, Berkeley and from 2004-2012 at Memorial Sloan-Kettering Cancer Center, New York. Follow-up studies show 20 percent of those treated live for at least three years with many living for 10 years and beyond, unprecedented results, according to the cancer center.
"I would like to keep on doing my research.so that this immune treatment could save more cancer patients", he said.
Prof Honjo said the award came "completely out of the blue" and "of course, I was very happy, delighted at the same time, but shocked".
Writing on his cancer centre's website, Allison said he was "honoured and humbled to receive this prestigious recognition".
Dr. Otis W. Brawley, a close friend of Allison's, said the Nobel committee usually waits about ten years to make sure a scientific discovery "sticks as being really important". The discoveries led to one of the decade's major advance in cancer therapy - drugs called checkpoint inhibitors.
Meanwhile, Allison left UC Berkeley in 2004 for Memorial Sloan Kettering research center in NY to be closer to the drug companies shepherding his therapy through clinical trials, and to explore in more detail how checkpoint blockade works.
The Nobel Prize in Physics is to be announced Tuesday, followed by chemistry.