Heidi,
We have had a lot of discussion on this, and it seems to me this is basically a futher example of my ideas on the role of the BBB.
If a person has a defective BBB, he is susceptible to toxins entering the brain, and this is one example of the possible toxins. If the BBB is not defective, you agree he will not show PD symptoms.

"These antibodies are ordinarily too large to pass through the blood-brain-barrier, and no damage occurs.
The BBB is compromised somehow. This may be due to age-related degradation or trauma or chemical exposure."

So a defective BBB is the key requirement to cause PD, not gluton sensitivity. I have a friend with celiac disease, and he certainly does not show any PD symptoms.
Other sources of damaging toxins have been identified, eg helicobacter pilori, but 50% of the population have this bacteria, but only the ones with a defective BBB get PD.

Back on my hobbyhorse of the defective BBB, remember that the permeability is variable. Stress causes a sudden widening with all the increase in symptoms that we know well. when the stress has passed, the BBB recovers somewhat, and our symptoms subside somewhat also. However, the BBB ages with time and becomes steadily less efficient, explaining why PD is largely an older person's disease.
I pointed out that the BBB would get to a permeability where dopamine could leak out of the brain into the bloodstream, once a critical permeability was reached. Also, I postulated carbidopa could leak in! This would prevent your brain being able to convert levodopa into dopamine.
A recent report confirms carbidopa can do exactly this.
http://www.ihop-net.org/UniPub/iHOP/...l?pmid=2753115
So carbidopa, like gluten and many other toxins can enter the brain and cause PD symptoms, but the overriding requirement is a defective BBB.
Ron