CD27 and systemic lupus erythematosus: Four B cell subpopulations including switched memory (SWM; IgD-CD27+), unswitched memory (USW; IgD+CD27+), double negative (DN; IgD-CD27-), and naïve (NAV; IgD+CD27-) B cells were detected in the peripheral blood of SLE patients (Fig. 1a).