Fig. 3.

Overview of the major features of asthma. Asthma is divided into two endotype: eosinophilic (Th2-high) and neutrophilic (Th2-low). Th2-high asthma is mainly related to Th2 lymphocytes and M2-polarized macrophages that can enhance eosinophilic inflammation via releasing inflammatory cytokines (e.g., IL-4, IL-5, IL-13, and TGF-β). Th2-low asthma is related to Th1, Th17 lymphocytes, and M1 macrophages that can release Th1, Th17, and M1 macrophage-related cytokines (IFN-γ, IL-17, and IL-1β/IL-6/IL-8), further promoting the neutrophilic inflammation. Asthma is also characterized by structural changes (remodeling), including epithelial damage, subepithelial fibrosis, and increased smooth muscle mass. Stressors (DAMPs, pollutants, allergens) can induce airway epithelial cells to release cytokines like TSLP, IL-25, and IL-33, promoting airway remodeling and inflammation.