CD4 and chronic obstructive pulmonary disease: Regulatory T (Treg) cells were a subgroup of CD4+lymphocytes that restrained the proliferation of other T cells and secreted anti-inflammatory cytokines, like IL-10 and TGF-β, providing the body with the necessary protection against over-activated immune responses by killing effector cells or APCs via releasing granzyme A and perforin.[25,26] Previous literature has reported in COPD patients, especially patients with long-term smoking, the level of Treg cells increased,[27] which was in line with our finding.