CD27 and systemic lupus erythematosus: This allows the identification of five well-defined B cell subsets (Figure 5B) i.e. mature naïve B cells (IgD + CD27-), Ab-secreting cells (ASC; IgD-CD27hi), switched memory B cells (Smem; IgD-CD27+), double negative memory B cells (DN; IgD-CD27-, a population of memory B cells described to be enhanced in lupus patients [30]) and non-switched memory B cells (NSmem; IgD + CD27+).