IL21R and systemic lupus erythematosus: 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).