Yes the digestive tract causes allergies. I had horrible head congestion. Tried all the meds, anti-histamines. Went to allergist. He said it was not connected to digestive tract. They tested me for allergies and said it was dust mites causing it all. Told me to take all this stuff, get special bed sheets.
Anyway, I clear up some of my problems a bit, and improve 90%+ in the head congestion/allergy department.