Diseases like multiple sclerosis and cancer could be slowed down or stopped, new research on genetics suggests.
Scientists at the University of Edinburgh said they have found that genetic "brakes" could lead to new treatments for these illnesses.
The growth of cells that can cause the conditions were previously thought to be controlled by a group of "master" genes, but the study discovered that there are hundreds of genes which interact with each other.
The variations in genes could also explain why people develop illnesses in different ways, the researchers said.
Professor David Hume, of the University of Edinburgh's Roslin Institute, who led the study, said: "This study has effectively shown us where the brakes are which could slow down or stop diseases like cancer and multiple sclerosis. We believe this could lead to treatments and cures for many diseases of the immune system."
The team said the findings show it may be possible to stop the growth of tumours by finding weak spots in the gene structure to allow the growth of healthy cells.
The findings are published in the Nature Genetics journal.