Conversely, serum serotonin levels correlated with BAL CD4+ T-cells (%) (p = 0.001, r = 0.92) and CD8+ T-cells (%) (p = 0.002, r = −0.90) in female COPD smokers, but not in male COPD smokers (p = 0.1, r = 0.46 and p = 0.1, r = −0.50, respectively). The gene discussed is CD4; the disease is chronic obstructive pulmonary disease.