The joint award to Allison and Honjo was given "for their discovery of cancer therapy by inhibition of negative immune regulation", the Nobel committee said.
Allison, who is a professor at the University of Texas MD Anderson Cancer Center, Houston, was studying a protein called CTLA-4 that inhibits a person's immune system by putting the brakes on the actions of T cells. Surgeon William Coley had developed an approach to treating cancer that involved injecting patients with a mixture of heat-killed bacteria in the hopes of stimulating the body's "resisting powers". A few years later, developers created drugs called Keytruda and Opdivo that release the PD-1 brake Professor Honjo discovered.
In the 25 years since they separately discovered "checkpoint inhibitors" in the immune system, their work has become the basis for a range of new cancer therapies. "It was one of those moments when we figured out that CTLA-4 was the brakes on the immune system". "For some patients, we see their tumours shrink or completely disappear and are effectively cured". Treatments that focus on the immune system or immunotherapy is widely acknowledged as one of the most exciting potential treatments for cancer.
The discoveries led to greatly improved therapies for skin cancer, as well as cancers of the lung, head, neck, kidney and liver.
In other Nobel Prize announcements, the physics prize will be announced Tuesday, followed by chemistry on Wednesday and the Nobel Peace Prize on Friday.
Last year's prize went to American scientists Jeffrey C Hall, Michael Rosbash and Michael Young for their research identifying genes and proteins that work with the body's internal biological clock, thereby influencing functions such as sleeping patterns, blood pressure and eating habits.
Honjo, who is now 76, told a news conference in Tokyo he was honored to get the Nobel, but that his work was not yet done.
"I'm still in sort of a state of shock, and this is all still sinking in", Allison said.
He said Allison's work a decade ago "really opened up immunotherapy" as a fifth pillar of cancer treatments, after surgery, radiation, chemotherapy and precision therapy. Their drug showed dramatic success in patients treated in 2012, including giving long-term remission to people with metastatic cancer.
Among those to have received such treatment is former USA president Jimmy Carter, who was diagnosed in 2015 with the skin cancer melanoma, which had spread to his brain. Experts previously thought that metastasis, when the cancer spreads to other organs and tissues, was untreatable, the Nobel committee's press release explains.