Evidence that the immune system attacks the brain in Parkinson's disease has finally been discovered.
Scientists have believed for many years that a malfunctioning immune system contributes to the condition, but have been unable to find conclusive proof until now.
Researchers at Columbia University examined the immune cells from 67 people with Parkinson's and 36 people in perfect health.
The study, published in the scientific journal Nature, indicated that the condition may trigger an autoimmune response, with health cells being mistakenly damaged and attacked.
Scientists working on the research believe that the immune system in those with Parkinson's might respond to the alpha synuclein protein.
This protein is understood to be recognised as a foreign invader, similar to a virus or bacterium, which means the immune system is activated and dopamine-producing brain cells come under attack.
Professor David Sulzer, an expert at Columbia University who was involved in the study, said: "The idea that a malfunctioning immune system contributes to Parkinson's dates back almost 100 years, but until now, no one has been able to connect the dots."
Dr Alessandro Sette, another scientist who worked on the research, added that the findings raise the possibility that an immunotherapy approach could be used to increase the immune system's tolerance for alpha-synuclein.
This, he said, could help to ameliorate or prevent worsening symptoms in Parkinson's disease patients.
The findings have been welcomed by Parkinson's UK, which said they lend weight to the "radical idea" that the condition may involve the immune system becoming confused and damaging people's cells.
However, Dr David Dexter, deputy director of research at the charity, stressed that scientists still do not know for sure why people develop Parkinson's and lose "precious" brain cells gradually over time.
"We still need to understand more about how the immune system may be involved in the complex chain of events that contribute to Parkinson's," he commented.
Nevertheless, Dr Dexter said the findings present an "exciting new avenue" to explore to help develop new treatments that may be able to slow or stop the condition in its tracks.
According to NHS estimates, approximately one in 500 people in the UK are affected by Parkinson's disease.
This means that about 127,000 people across the country are currently living with the condition, with men more likely to get Parkinson's disease than women.
The NHS points out that the majority of people with Parkinson's typically start developing symptoms when they are 50 or over.
However, figures suggest that about one in 20 people with the condition first experience symptoms when they are under 40.