New nomenclature of allergic diseases and hypersensitivity reactions

The review presents the current understanding of the mechanisms of initiation and development of hypersensitivity reactions triggered by external factors, based on the new 2023 nomenclature of allergic diseases proposed by the European Academy of Allergy and Clinical Immunology. It details the new concept of hypersensitivity, which systematizes previous classifications and summarizes the new data on the etiology and mechanisms of inflammatory reactions in comparison with diverse immune response variants, based on the concept of disease phenotypes and endotypes. The today’s concept of allergic reactions covers several types: those associated with antibody formation (I–III), cell-mediated reactions (IVa–c) and the Pichler concept, tissue-dependent ones caused by the epithelial barrier defects and metabolism-induced immune dysregulation (V–VI), and direct immunoinflammatory reactions to chemicals (VII). In the updated classification, both adaptive and innate immune responses actively participate in initiation and effector phases of the hypersensitivity reactions. The review highlights the role of regulatory and effector lymphocyte subpopulations and innate lymphoid cells in various types of hypersensitivity. It also reports a pivotal role of epithelial barrier dysfunction in many allergic diseases and the effect of viral infections on their courses. The significance of the metabolic dysregulation and its impact on the immune response is emphasized. According to the nomenclature of allergic reactions, several pathophysiological mechanisms may underlie the development of a same disease. Notably, various hypersensitivity mechanisms involve most cellular and humoral factors, reaffirming the universal biological nature of immune response. As awareness of the new concept grows and clinical and experimental data accumulate, the updated classification of the hypersensitivity types will contribute to a theoretically substantiated and practically effective framework for the prevention, accurate diagnosing, and improved treatment of allergic diseases.