Interesting thread this. I used to be allergic to dogs when I was very young. Not just a bit but when my parents hired a car I had an attack, and had to be admitted to hospital, all because a previous hirer had had a dog in there 3 weeks previously. So if I go on a plane that has had a guide dog on I would have had serious problems but there is nothing that can be reasonably be done about it. I, and my parents would just have to live with it, even when encumbered with inhalers, tablets etc.
A friend of mine, his daughter has a peanut allergy and suffered a reaction when in the air. The crew were great, they knew that there was a peanut allergy sufferer on the plane, and didn't sell any on that flight. But she still had a reaction. Why? Probably a previous person on the plane had some, wiped their hands on the arms of the seat, and she picked it up from there.
Sometimes, in spite of the bast of intentions, you really cannot stop these things happening.