In “body-first” Parkinson's disease, pathology starts in the gut and migrates to the brain. The route was unknown. The assumption: misfolded alpha-synuclein spreads along nerve fibers, climbing the vagus nerve from gut to brainstem like a vine.
The actual route is immunological. Gut macrophages engulf misfolded alpha-synuclein — their job, their function, exactly what they're supposed to do. But they don't destroy it. The engulfment corrupts them, and they activate T cells in the enteric nervous system. These T cells expand, then traffic to the brain, initiating neurodegeneration.
The through-claim: a half-completed immune response is more dangerous than no response at all. The macrophages that eat alpha-synuclein become the disease's delivery system. If they fully degraded the misfolded protein, no signal would propagate. If they ignored it, the protein would remain local. Instead, they take it up without neutralizing it — engulfment without destruction — and the partial processing generates an inflammatory signal that recruits T cells as secondary vectors.
The defense system becomes the delivery system. Depleting gut macrophages or blocking T cell migration in mouse models reduced neurodegeneration — confirming that the immune response itself, not just the pathological protein, drives the disease's spread.
The irony is architectural: the immune system is the body's fastest long-distance communication network. Once corrupted, it becomes the fastest long-distance disease network for exactly the same reason — it has the infrastructure to move signals from gut to brain quickly and efficiently. The capability that makes it protective is the capability that makes it destructive. The taxi doesn't care what it's carrying.