CD27 and COVID-19: To determine the involvement of CD19+ B cells in the pathogenesis of COVID-19, circulating CD19+ (total B cell), CD19+IgD+CD27− (naive B cell), CD19+CD27+ (total memory B cell), CD19+IgD+CD27+ (non-switched B cell), CD19+IgD−CD27+ (switched B cell), CD19+CD38++CD138+ (plasma cell) and CD19+CD38+CD24− (plasmablast) were analyzed by flow cytometry (Figure 3A).