2.3. Collecting induced sputum

Participants were asked to induce sputum production by inhaling a 4.5% sodium chloride aerosol solution under the supervision of a respiratory clinician. Sputum samples should be processed within 2 h after collection. The sputum bolt was then dissolved with 0.1% DL‐Dithiothreitol (DTT; Sigma), a volume four times its weight. ³⁵ The dissolved solution was filtered through a cell screen and centrifuged (1000g 5 min, Eppendorf), the sputum cells in the sediment were collected in 1 mL TRIzol (Invitrogen) for RNA extraction and the sputum supernatant were stored in −80°C refrigerator for enzyme‐linked immunosorbent assay (ELISA) detection. Besides, other clinical information including FeNO, blood EOS%, and serum IgE were collected.