A 'switch' that creates immune cells which hunt and kill cancer has been identified.
Researchers say the natural killer cells hunt and destroy foreign cells, including cancer cells that can spread and form tumours.
They work by delivering toxic granules into cancerous or infected cells, causing them to explode and die.
Two million people are currently living with the disease in the UK alone.
Previous studies of the cancer-killing cells have found them to be particularly effective at destroying breast, colon and melanoma cancer cells.
Yet while scientists knew a switch or regulator was essential for how these immune cells developed, nobody had any idea about how it worked.
Now, researchers have found a protein called ID2, which works by allowing natural killer cells to become responsive to growth factors in the blood.
These natural killer cells are reliant on one particular growth factor, IL-15.
If this is removed, the natural killer cells will die.
The exciting new findings, published in the journal Immunity, mean other cells could be transformed into natural killer cells, even when the switch is missing, simply by supplying more growth factor to a person's bloodstream.
And the findings offer hope to those with other health conditions.
The researchers say the switch could be manipulated to fight viral infections or help patients whose immune systems are not properly developed.
Furthermore, the switch could be 'turned off' to produce fewer natural killers when they may be damaging, such as in bone marrow transplant patients and toxic shock syndrome sufferers.
Author Dr Nick Huntington, of the Walter and Eliza Hall Institute in Australia, said: "This is an exciting discovery because previous research has shown that these natural killer cells are really potent in killing tumours: breast and colon cancer and melanoma cells.
"If we can give an advantage to natural killer cells by boosting their activity or numbers or survival in the body then we can try to win that fight against cancer.
"We've basically identified how natural killer cells are born and how they're maintained in our body. Now we know how to best keep them fit and healthy to keep us healthy."
Dr Huntington added: "The real paradigm shift is that we can now make natural killer cells appear even when this switch is missing, purely by supplying more growth factor to the specific environment - we can push cells to become natural killer cells. It's a really novel biological discovery."