Allison, a professor at the University of Texas MD Anderson Cancer Centre, studied a protein that functions as a brake on the immune system and realized the potential for unleashing immune cells to attack tumours if the brake could be released.
Allison, an American immunologist, conducted a study on a protein that functions as a brake on the immune system.
Honjo separately discovered a new protein on immune cells and eventually found that it also acts as a brake.
The discoveries by Allison, 70, and Honjo, 76, "absolutely paved the way for a new approach to cancer treatment", Dr. Jedd Wolchok, chief of the melanoma and immunotherapeutics service at the Memorial Sloan Kettering Cancer Center in NY, told The Associated Press.
Therapy developed from Honjo's work led to long-term remission in patients with metastatic cancer that had been considered essentially untreatable, the Nobel Assembly said.
Allison's research was conducted at the University of California, Berkeley. For many scientists, he said, a driving motivation "is simply to push the frontiers of knowledge".
Antibodies against PD-1 have been approved by the US Food and Drug Administration as an investigational new drug for the treatment of cancer.
"Until the discoveries made by the 2018 Medicine Laureates, progress into clinical development was modest".
Charles Swanton, chief clinician at the charity Cancer Research UK, said the scientists' work had revolutionised cancer and immunotherapy. "The number of different types of cancers for which this approach to immunotherapy is being found to be effective in at least some patients continues to grow". It was because of a sexual misconduct scandal that led to the decision was the Nobel assembly.
The winners of this year's physics prize will be announced on Tuesday, followed by the chemistry prize on Wednesday. The prizes for achievements in science, literature and peace were created in accordance with the will of dynamite inventor and businessman Alfred Nobel and have been awarded since 1901.
"Cancer kills millions of people every year and is one of humanity's greatest health challenges", the Nobel committee said on Twitter.