B cells from SLE patients have lower surface expression of the CD19/CD21 receptor complex but increased expression of other receptors including CXCR4 and IL-21R (9, 43, 44); a number of these differentially expressed proteins are thought to drive disease pathogenesis (45–47), while others are considered to be biomarkers that reflect immune dysregulation in these patients (48, 49). Here, CD19 is linked to systemic lupus erythematosus.