Some studies have shown a decrease in the frequency of NK cells expressing CD158e1 in the peripheral blood among healthy smokers and COPD smokers compared to healthy nonsmokers [29], along with reduced expression of the inhibitory CD94 receptor on peripheral NK cells and BALF NK cells in COPD smokers relative to nonsmokers [30]. This evidence concerns the gene KIR3DL1 and chronic obstructive pulmonary disease.