I adopted my dog Scooby from the dog pound years ago. Shortly after adopting him I found he has an allergy to almost everything! He's allergic to fleas. I started giving him brewer's yeast when I discovered he is allergic to yeast!
Scooby recently had a bacterial ear infection that moved to the rest of his body. It made his hair fall out in clumps. It was small patches so it was hard to tell what was going on at first and it appeared almost overnight. The bacteria from his ears had moved to the rest of his body making "scabs". Those scabs would "burst" and the hair would fall out. The vet put him on antibiotics for several weeks and gave me an ear rinse/ear antibiotic treatment to do. He's gotten better.
Scooby has to take benadryl on a somewhat regular basis. I don't give it to him everyday but on occassions of "breakouts". He's already up to one pill and he's only 35lbs! Usually, I have to take him to the vet for a full dose of antihistimines for his outbreaks.
I am afraid the worst has already happened to Scooby as a result of his allergies. I think he may have had a "stroke" due to an undiagnosed fever from an possible flea reaction. We never knew the exact cause. So it is important to always keep an eye out for signs of allergic reactions. Scooby's skin turns "pink" and he scratches on anything and everything. He also will feel extremely "warm" to the the touch.
Shampoo's have had a limited effect on Scooby. I do try to use "soothing" shampoos. However, I tend to really just hose him off with plain water in between baths or exposure to other animals. He's got to use frontline to keep the fleas off of him. One bite and we are at the vets office!
I hope the allergy testing helps. I wish I had found out with Scooby. Unfornately, it has been with trial and error with is allergies.