CD27 and COVID-19: From 10 COVID-19 patients with serum antibodies to the S1 subunit of the SARS-CoV-2 spike protein (Figure S1A; Table S1), we isolated two populations of single cells from peripheral blood mononuclear cells with fluorescence-activated cell sorting (FACS): CD19+CD27+CD38+ antibody-secreting cells (ASCs) reflecting the overall humoral immune response and SARS-CoV-2-S1-labeled CD19+CD27+ memory B cells (S1-MBCs) for characterization of antigen-specific responses (Figures S1B and S1C).