CD209 and COVID-19: To investigate the variation of DC status and DC-SIGN expression in COVID-19 cases, in this study, we systematically studied the transcriptome in single cells in the upper airway (nasopharynx/pharynx samples), lower airway (bronchoalveolar lavage fluid (BALF) samples) and peripheral blood (peripheral blood mononuclear cell (PMBC) samples) of COVID-19 patients with different severity levels.