Three things.
1 - My younger daughter had a type of hives called cholinergic uticaria which is hypothalamic - comes from any elevation of body temperature - exercise, happiness, excitement, stress (excitement is a form of stress): http://itsnotmental.blogspot.com/2008/02/stress-immune-re...
2- Different types of allergic responses and not all chemicals, etc are tested:
My young niece had hives from the laundry detergent and we had to switch to types used for baby items.
It took me so long to find out saccharin and aspartame caused me hives. I was keeping a food diary, but had not thought about medicines. I was trying to avoid saccahrin and it was in my cough medicine, THEN, it was in the liquid Benadryl I was taking for the hives! I ended up in the ER with a weird type of allergic reaction I had never heard of - allergic arthritis.
Then, I started reacting to more and more foods.
Anyway, I found we do have different types of allergic reactions - IgE, IgG, and IgA. We are having an epidemic of reactions - not just in the US. Europe is way ahead of us recognizing what is going on with gluten and dairy, for instance (especially the gluten issue) - You may want to look at http://www.enterolab.com for testing for IgA testing.
Here is more:
http://itsnotmental.blogspot.com/2011/02/we-have-sick-chi...
3- One solution, of course is to avoid whatever causes the hives. But in the meantime, what helped with both types of hive was a combination of antileukotrienes (Singulair) and antihistamines (Allegra, and/or Zyrtec). These were administered under the care of an allergist. The only side-effect I believe she had from that were dry eyes for which she used liquid tears.