Allergen immunotherapy (AIT) with specific allergen is currently regarded as the only causal treatment of allergic diseases. Efficacy of AIT has been confirmed for both inhalant allergy as well as for mild to moderate atopic asthma. Allergen immunotherapy modifies multiple functions of Th2, Treg, Breg, ILC (and the others) cells engaged into pathogenesis of allergic inflammation. The most frequent forms and ways of application of AIT are subcutaneous AIT and sublingual AIT. According to the current guidelines 3-years lasting treatment with AIT is regarded as sufficient to achieve expected therapeutic outcomes.