The clinical expression of the most common allergic diseases reflects allergic inflammation and underlines that inflammation is the main target of anti-allergic therapies. Allergen specific immunotherapy (AIT) has a recognized impact on allergic inflammation, which persists after its discontinuation, and is the only therapy able to modify the natural history of allergic march. The traditional, subcutaneous route of administration is effective in altering the phenotype of allergen-specific T cells, switching from a Th2-type response, characterized by the production of IL-4, IL-5, IL-13, IL-17, and IL-32 cytokines to a Th1-type response. This immune deviation is related to an increased IFN-gamma and IL-2 production as well as to the anergy or tolerance of Th2, the latter related to the generation of allergen-specific T regulatory (Treg) cells, which produce cytokines such as IL-10 and TGF-beta. Anti-inflammatory mechanisms observed during sublingual IT with high allergen doses proved to be similar compared to subcutaneous immunotherapy. Recent data obtained in biopsies clearly indicate that the pathophysiology of the oral mucosa, and in particular mucosal dendritic cells, plays a pivotal role in inducing tolerance to the administered allergen.