According to Dr. Laura Robinson, DVM, there are three main reasons for allergies: flea allergies, food allergy, and environmental (grass, humans, yes dogs can be allergic to you!). It can even be a mixture of those three. With food allergies, symptoms will be appear earlier in their life, whereas fleas and environmental allergies can appear anytime.
Common symptoms of allergies include licking of the paws, scratching and chewing different spots, as well as red skin and hives. Common ear infections can also signify allergies.
Purebreds seem to have worse allergic symptoms, the main breeds being boxers, golden doodles, labs, French bulldogs, English bulldogs, pit bull breeds, and West Highland terriers. However, mixed breeds can have them too.