ATOPIC ALLERGIC DISEASES Allergic Asthma Allergic asthma is caused by the same allergens as allergic rhinitis in atopic individuals, pathogenically linked to chronic allergic inflammation in the lung based on atopic sensitization and Th2 polarization, bronchial hyperresponsiveness, edema, and hypersecretion, and typically manifests itself as wheezing, coughing, and bronchial obstruction. As a rule, asthma may begin at the age of 4 years and older having a significant long-term impact on the quality of life for patients. Asthmatic attacks can be triggered not only by causative allergens but also by respiratory infection, exercise, cold air, drugs like aspirin, or pollutants like sulfur dioxide and phthalates. Asthma is associated with changes in the structure of the large and small airways and airway remodeling. The airway remodeling is characterized by goblet cell hyperplasia with an excess of mucus secretion, smooth muscle cell hyperplasia with bronchoconstriction, and airway hyperresponsiveness. Mast cells, eosinophils, and neutrophils play an important role in the pathogenesis of allergic inflammation in asthma. Asthma is inherent in heterogeneity and complexity in clinical and immunologic features and division into some endotypes. Both two main endotypes, Th2-high/eosinophilic and Th2-low/Th17/neutrophilic, include patients with mild, moderate, and severe asthma. However, patients with Th2-high/eosinophilic endotype have a milder course of COVID-19. An important lung function parameter in asthma is the ratio of the 1st second forced expiratory volume (FEV1) to forced vital capacity (FVC) (Tiffeneau Index), which diminishes in cases of bronchial obstruction: lower 0.75-0.80 in adults and 0.90 in children. Fractional exhaled nitric oxide (FeNO) is a widely exploited parameter assessing the level of chronic allergic inflammation in asthma. In addition, the measurement of nitric oxide, an atypical gasotransmitter, enables the prediction of response to inhaled corticosteroids (if FeNO > 35 ppb) and the reversibility of bronchial obstruction. Depending on response to treatment, asthma may be controlled and uncontrolled. Therapy for the disease includes inhaled corticosteroids, leukotriene receptors antagonists, and biologics, β agonists if required, and allergen-specific immunotherapy (AIT). ©V.V.Klimov