Serum serotonin levels correlated positively with BAL CD4+ T-cells (%) (p = 0.001, r = 0.65) and negatively correlated with BAL CD8+ T-cells (%) (p = 0.001, r = −0.65) in COPD smokers (Figure 6A–C); however, levels did not correlate with blood CD4+ T-cells (%) or CD8+ T-cells (%) (Figure S6). Here, CD8A is linked to chronic obstructive pulmonary disease.