Figure 2. Contribution of CD4⁺ Th cells to airway inflammation. APCs including DCs process antigen (allergens/viruses) and present them on naive T cells, which subsequently induces the differentiation of naïve T cells into Th1, Th2, Th17, and Tregs cells, depending on the microenvironment orchestrated by the cytokine milieu. Following exposure to allergens, the Th2 response facilitates the accumulation of eosinophils in the airway via production of IL-4, IL-5, IL-9, and IL-13, whereas Th1 response is linked to mixed (eosinophil-neutrophil) inflammation via induction of IFN-γ, IL-8, TNF-α. Th17 cells mediate neutrophilic inflammation that is frequently observed in severe refractory asthma. CD4⁺Foxp3⁺ Tregs secreting IL-10 and TGF-β coordinate the tolerance to inflammation and balance of Th1/Th2 response.