CD27 and COVID-19: Moreover, based on analyzing the key B cell subsets we found that critically ill patients were characterized by elevated percentage of CD27+CD38hi, CD21−CD11c−DN3 and IgD−CD27+ memory B cells, whereas patients with severe COVID-19 had elevated percentage of CD21−CD11c+ DN2, atypical CD27-negative memory B cells as well as CD11c+IgD+CD38−/+ activated naïve B cells [15].