Immunotherapy for hay fever due to ragweed, grass, birch, and mountain cedar pollens and for asthma due to ragweed, grass, house dust mite, and cat dander allergens relieves symptoms in controlled studies. Clinical improvement is specific for the allergen employed, depends on the dose administered, and may relapse six months to a year after injections are stopped. Immunological changes include a rise in IgG antibodies, an early rise and later decline in IgE antibodies, sometimes a rise in secretional antibodies, reduced basophil reactivity and sensitivity, and reduced lymphocyte responsiveness to allergens.