The immune system may have a key role in the development of Parkinson's disease, say US researchers.
In a 20-year study of 4,000 people, half with Parkinson's disease, the team found an association between genes controlling immunity and the condition.
The results raise the possibility of new targets for drug development, Nature Genetics reports.
Parkinson's UK said the study strengthened the idea that immunity is an important driver of the disease.
The team were not just looking for a genetic cause of the disease, but also considered clinical and environmental factors.
During their search, they discovered that groups of genes collectively known as HLA genes are associated with the condition.
These genes are key for the immune system to differentiate between foreign invaders and the body's own tissues.
In theory, that enables the immune system to attack infectious organisms without turning on itself - but it is not always an infallible system.
The genes vary considerably between individuals.
Some versions of the genes are associated with increased risk or protection against infectious disease, while others can induce autoimmune disorders in which the immune system attacks the body's own tissues.
Multiple sclerosis has already been shown to be associated with the same HLA genetic variant seen in the latest study in Parkinson's disease, the researchers said.
It was already known that people who take anti-inflammatory drugs, such as ibuprofen, have a decreased risk of developing Parkinson's disease, which also supports the idea that the immune system has a role in the disease.
But this protective effect is not the same for everyone, probably because of genetic differences.
With better understanding of the link between Parkinson's disease, immunity and inflammation, it may be possible to design more effective drugs for treating the condition, the researchers said.
"Over the years, there have been subtle hints that immune function might be linked to Parkinson's disease," said study leader Dr Cyrus Zabetian, associate professor of neurology at the University of Washington.
"But now we have much more convincing evidence of this and a better idea of which parts of the immune system might be involved."
He added: "Our results also pointed to several other genes that might play a role in developing Parkinson's disease, and these findings need to be confirmed, so we have a lot of work ahead of us."
Dr Kieran Breen, director of research at Parkinson's UK, said the work provides additional evidence of the role of inflammation in the development of Parkinson's disease.
"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."
He added: "This research, combined with Parkinson's UK funded research at Oxford University into the role of inflammation, may lead to the development of new drug treatments for the condition."