Allergen immunotherapy is the practice of administering gradually increasing quantities of an allergen extract to an allergic subject to ameliorate the symptoms associated with subsequent exposure to the causative allergen.

Guidelines and indications for immunotherapy with inhalant allergens have been published by the World Health Organization (WHO), the European Academy of Allergy and Clinical Immunology (EAACI), and the British Society for Allergy and Clinical Immunology.

Controlled studies demonstrate that allergen immunotherapy is effective for patients with allergic rhinitis/conjunctivitis (hay fever), allergic asthma and stinging insect hypersensitivity. Immunotherapy is effective treatment for many pollen species, including grasses, trees, ragweed, etc., house dust mite and moulds and it is the only treatment available that may alter the course of the allergic illness. Whereas symptomatic treatment (antihistamines, bronchodilators, etc.) only relieve the symptoms of the disease, allergen immunotherapy can improve the disease in the long term.

Immunotherapy can be administered both by subcutaneous (shots) or sublingual way.