CD4 and chronic obstructive pulmonary disease: CD4+ T cells differentiate into different functional T cell subsets upon activation, including Th1, Th2, Th9, Th17, Th22, Tfh, and regulatory T (Treg) cells, and CD8+ T cells also differentiate into diverse subsets, including Tc1, Tc2, Tc9, Tc17, Tc22 cells, Tfcs, and suppressive CD8 Tregs (48), of which Th1/Tc1, Th2/Tc2, Th17, Treg are closely associated with COPD (49).