Probably the second most common skin condition I see in dogs (after flea-related problems) is allergic skin disease, or atopy.
Dogs can develop an allergic reaction to any number of things in the environment, or, less commonly, to their food. Common indoor allergens include house dust mites, detergents and carpet cleaning products. Common outdoor allergens include grasses and pollens. Food allergens include beef, pork, dairy products and wheat. And of course fleas themselves can cause allergic skin disease in some unlucky dogs.
An allergic reaction is caused when the immune system makes antibodies to common substances instead of to those which are “foreign” to it. The antibodies cause mast cells in the skin to release chemicals like histamine, which cause irritation and inflammation.