Objective The aim of this review is to provide an overview of the current knowledge on the mechanisms of allergen immunotherapy based on the recent publications and clinical trials. Data sources PubMed literature review. Study selections In this review, we focus on diverse mechanisms of AIT and provide an insight into alternative routes of administration. Additionally, we review and discuss the most recent studies investigating potential biomarkers and highlight their role in clinical settings. Results Successful allergen-specific immunotherapy (AIT) induces the reinstatement of tolerance toward allergens and represents a disease-modifying treatment. In the last decades, substantial progress in understanding the mechanisms of AIT has been achieved. Establishment of long-term clinical tolerance to allergens engages a complex network of interactions, modulating the functions of basophils, mast cells, allergen-specific regulatory T and B cells, and production of specific antibodies. The reduction of symptoms and clinical improvement is achieved by skewing the immune response away from allergic inflammation. Conclusion Although the complex nature of AIT mechanisms is becoming more clear, the need to discover reliable biomarkers to define patients likely to respond to the treatment is emerging.