CD19 and COVID-19: We regarded the frequency of peripheral blood circulating B cells and found that both groups of patients with COVID-19 had significantly lower concentrations of CD19+ cells compared to those in the healthy donors (108.3 (76.0;195.1) cells/μL in severe COVID-19 and 84.9 (44.4;188.2) cells/μL in moderate COVID-19 vs. 208.9 (170.6;311.2) cells/μL in HD, with p > 0.001 and p > 0.001, respectively).