I flipped through an interesting book many years ago - I can't recall the title, but it was written by a medical doctor who spent many years poking around the phenomenon on a case-by-case basis. He was more concerned with treatment than science, so no large scale studies, but there was a wealth of anecdotes. It may possibly have been "The Food Allergy Book"[0] (the Amazon description makes it sound at bit nutrition-woo ish, and there was a certain element of that, but his approaches were actually scientific with blind tests and controlling out variables and so on).
There wasn't really much focus on "root" cause, but there were some interesting patterns. Two stand out in my memory: excessive consumption of a particular thing seemed to instil the "allergy", and once present would be triggered even if the "allergen" were administered intravenously without the patient's knowledge. In one case a man who had access to cheap eggs had them daily in large quantities for years - when finally seeking treatment for some very general symptom (migraines?), the hospital took him off his egg-diet. Thereafter, even the tiniest amount of intravenously administered egg would cause drastic symptoms (what symptoms I can't recall either, but may even have been as strange as violent episodes).
I realize this isn't the rigorous science you were looking for, and I apologise for the vagueness of my memory, but if it is indeed the same thing your wife experienced you can probably rule out "learned" correlations.
There wasn't really much focus on "root" cause, but there were some interesting patterns. Two stand out in my memory: excessive consumption of a particular thing seemed to instil the "allergy", and once present would be triggered even if the "allergen" were administered intravenously without the patient's knowledge. In one case a man who had access to cheap eggs had them daily in large quantities for years - when finally seeking treatment for some very general symptom (migraines?), the hospital took him off his egg-diet. Thereafter, even the tiniest amount of intravenously administered egg would cause drastic symptoms (what symptoms I can't recall either, but may even have been as strange as violent episodes).
I realize this isn't the rigorous science you were looking for, and I apologise for the vagueness of my memory, but if it is indeed the same thing your wife experienced you can probably rule out "learned" correlations.
[0] http://www.amazon.com/The-Food-Allergy-Book-Walsh/dp/0963154...