After classifying small (<2 mm diameter) airways from COPD patients as SIgA-deficient (SIgA-) or SIgA-replete (SIgA+) based on immunostaining for SIgA on the luminal surface as previously described,12 we determined the number of CD8+ and CD4+ T lymphocytes present in the airway wall (Fig. 1a). This evidence concerns the gene CD4 and chronic obstructive pulmonary disease.