Some dogs are highly allergic and can develop allergies to grasses, pollen, dist mites, and mold. It is not uncommon for these dogs to also be allergic to flea bites. Other common causes of allergies in dogs are food allergies. Least commonly, dogs can develop a contact allergy usually the result of a plant that has touched the skin. Antihistamines don't work very well for most dogs and identifying airborne allergies is very difficult without allergy testing.