Reducing exposure to the causative allergen relieves allergy symptoms. But how do you do that? Manzano and colleagues from IAF/ZAUM made a study, where they used commercially available air purifiers to eliminate fine particles from indoor air, to which the allergens stick. They could reduce the allergen load by about 80% (depending on allergen).
Of course there are other ways to reduce allergen exposure: encasings of your matress for house dust mite, wash the cat, or remove the dog from the home, and many more. However, the success of these methods is sometime limited or not so easy to do. Using an air purifier is a simple additional method to the repertoire of allergen reducing measures.