We compared the frequencies of B cell subsets (including IgD+CD27− naïve B cells (naïve B cells), IgD+CD27+ unswitched memory B cells (UM B cells), IgD−CD27+ switched memory B cells (SM B cells), total CD27+ memory B cells, IgD−CD27− double negative B cells (DN B cells), CD19+CD20−CD27hi plasmablasts (plasmablasts), and CD24hiCD27+ B cells (B10 cells)) between AOSD patients and HCs. Here, CD27 is linked to adult-onset Still disease.