For decades, scientists have suspected that the immune system is involved in Parkinson's disease but didn't know how because the body's defenses don't typically target neurons. Researchers now report that immune cells may be attacking a protein that builds up in the brain when someone has the disease. The finding suggests an avenue for new treatment by blocking the immune response with drugs.

What they did: Researchers analyzed blood from 67 patients with Parkinson's disease and 36 healthy people, and found that in people with the disease the body's immune T cells recognized neurons displaying specific fragments of a protein called alpha-synuclein, which accumulates in the brain when someone has Parkinson's. In healthy people, there was no response.

How it might work: The reaction to specific pieces of the protein is key - it could explain why immune reaction is localized to particular neurons. The immune system may recognize alpha-synuclein as foreign and try to clear it, in the process destroying dopamine-producing neurons where the protein accumulates. Those neurons are at the center of Parkinson's - depleting dopamine causes the disease's hallmark tremors. Most neurons don't produce the molecules that the immune system recognizes and then attacks - but the dopamine-producing ones do.

"It's a novel idea. If it holds up, it will shift the paradigm for understanding the disease," says NIH's Mark Cookson, who wasn't involved in the study.

Big question: Does the immune system target these neurons or is the reaction is inadvertent? That would indicate whether the reaction can be blocked with drugs - something the researchers plan to test.