Some people may be more susceptible to Parkinson's disease because of glitches in their immune systems, scientists say.
A study of patients with the brain disorder found they were more likely than healthy people to carry a gene mutation thought to disrupt how the immune system works.
The finding suggests some cases of the disease could be caused by the immune system running amok and attacking healthy tissues or failing to fight infections that leave people susceptible to the condition.
The study is the first to use evidence from the human genome to confirm the long-held suspicion that the immune system plays a role in the disease.
"People have speculated about a link between the immune system and Parkinson's disease for some time and this study suggests that the link is real," said Cyrus Zabetian, a co-author on the study and neurologist at the University of Washington in Seattle.
Parkinson's disease is caused by a steady dying-off of brain cells that produce a chemical called dopamine. One in 500 people in Britain have the condition, which causes tremors and difficulty with movement. Most are diagnosed after the age of 60.
Scientists led by Haydeh Payami, a professor of genetics at New York state's Wadsworth centre, compared the genetic make-up of 2,000 patients with late-onset Parkinson's disease and 1,986 healthy volunteers. They found four mutations that were more common in the patients than the control group.
Three of the mutations are already known to raise the risk of Parkinson's disease, but the fourth had not been linked to the condition before. The newly discovered mutation is in a region where immune system genes are known to reside.
The finding might help to explain why drugs that suppress the immune system, like ibuprofen, appear to protect against Parkinson's disease. Nicotine and caffeine are also thought to delay the onset of the disease.
Payami said that building up a genetic picture of Parkinson's disease would help doctors personalise their treatment of patients. The study is published in the journal, Nature Genetics.
Kieran Breen, director of research at Parkinson's UK, said the finding is further evidence for the role of inflammation in Parkinson's. "Previous studies have shown that inflammation does occur in the brains of people with Parkinson's, but identifying the potential factors that may lead to the death of nerve cells has been difficult.
"We know already that some people are more susceptible to getting Parkinson's due to their genetic makeup. This study also points to some genes that may be involved."