CD4 and chronic obstructive pulmonary disease: The large body of experimental data on lymphocyte trafficking [35] implies that the decreases we found in blood CD4+ and CD8+ T cells during AE-COPD reflect recruitment both of naïve and memory lymphocytes into regional lymph nodes and organized lymphoid tissues (which develop within lung parenchyma in advanced COPD [33, 36]) and of effector-memory cells to the lungs.