Mass cytometry (STAR Methods; Data S3) demonstrated significant lymphopenia in COVID-19 with reduced overall frequency and number of B cells, predominantly naive B cells, but an increase in terminally differentiated plasmablasts (significantly higher than in sepsis) and a relatively high proportion of CLA+ plasmablasts (Figures 1B, 4A, and S6I). Here, SELPLG is linked to lymphopenia.