IL4 and chronic obstructive pulmonary disease: Inflammatory ILC2 (iILC2) entering the lung tissue then upregulate the expression of major histocompatibility complex class 2 (MHC-II) or secrete IL-13 and IL-4, providing a microenvironment for Th2 cell differentiation, promoting Th2-type immune responses, and transferring inflammation from the gut to the lungs, participating in the pathogenesis of COPD (98).