CD4 and chronic obstructive pulmonary disease: Both the cell lines develop from the common naive T CD4+ subset but functionally antagonize each other and maintain the critical balance of immune homeostasis.155 Studies have reported an increased proportion of circulating Th17 cells in COPD patients.152,156–158 A study reported a perturbed Th17/Treg balance in COPD with a shift towards pro-inflammatory response in patients showing acute exacerbations.152 Another study found that a Th17/Treg imbalance existed in the bronchoalveolar lavage fluid of mice after chronic cigarette smoke exposure.156