Vet Dr. Patrick McHale, DVM from http://www.dog-health-guide.org/canineskinallergy.html describes the causes and provides tips for treating dog skin allergies, dermatitis and atopy. Dogs often get contact allergies or dermatitis such as poison ivy, skin reactions from parasites such as fleas, and inhaled seasonal allergies from pollen (called atopy). An allergic reaction to fleas is the most common form of skin allergies in dogs.
Dogs can get itchy just in the presence of fleas or from the flea saliva itself. Fleas are great at hiding on the dog, and even a few fleas can cause an allergic reaction. Because this is the most common form of canine or puppy skin allergy, a veterinarian will eliminate this as a possible cause before investigating other reasons for the problem.
Season allergies are sometimes easier to determine since they only start at a particular time of year such as spring, summer or fall. Contact allergies also often appear when a dog might be outside for longer periods of time, such as when poison ivy or oak causes a skin reaction.
If dog food allergy is suspected after investigating all of the other possible reasons, then the veterinarian will put the dog on a special diet (called an elimination diet) that reduces the diet down to two ingredients, a simple carbohydrate such as rice and a simple protein such as chicken. A dog would need to stay on this diet for 4 to 6 weeks to see if symptoms subside. See the web page at the beginning of this description for details on which parts of the body are affected, pictures and symptoms associated with each of the skin allergies in dogs.