Atopic dermatitis is an inflammatory, chronic skin disease associated with allergies. In fact, this is the second most common allergic skin disease in dogs. These allergic reactions can be brought on by normally harmless substances like grass, mold, house dust mites, and other environmental allergens.   Dogs normally show signs of the disease between 3 months and 6 years of age, though atopic dermatitis can be so mild the first year that it does not become clinically apparent before the third year. (from PetMD)