IL4 and chronic obstructive pulmonary disease: Although pro-inflammatory CD8+ T cell pool (mainly including CD8+ IFN-γ+Tc1, CD8+ IL-4+Tc2, and CD8+ IL-17+Tc17) and anti-inflammatory CD8+ T cell pool (mainly including CD8+ Foxp3+Treg, and CD8+ IL-10+Tc10) have been considered to be involved in COPD pathogenesis, existing studies regarding this association vary widely in their designs, population sizes and experimental methods.