Allergies in dogs is perhaps more common that you may realise. We have a Web Page dedicated to helping dogs with allergies.
Allergies can cause hair loss, flaky and itchy skin, behavioural problems, biting and chewing causing self mutilation. reddening of the skin, eye infections, ear infections and other symptoms. The only reliable way to know for sure what your dog is allergic to is to have a blood test done. This is expensive but in my opinion, having a severely allergic rescue spaniel ourselves, it is well worth it as potential allergens can then be eliminated from the environment as far as possible.