The purpose of this study is to determine whether there is a statistical association between the changes from baseline in the levels of two cytokines interleukin (IL)-17A and IL-6 in the sputum of patients with chronic obstructive pulmonary disease (COPD) and the severity of acute exacerbations of COPD (AE-COPD). These sputum cytokine levels are taken as measures of the adaptive immune response (IL-17A) and the innate immune response (IL-6), respectively. Sputum will be collected either spontaneously or will be obtained by induction; cytokine levels will be measured by ELISA. The primary analysis, comparisons of sputum cytokine levels between clinical states, will be done using random effects modeling.