CR2 and systemic lupus erythematosus: Over the past decade, B cells characterized by a low expression or absence of CD21 (CD21lo B cells) gained interest due to their expansion in the peripheral blood of patients with autoimmune disorders such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and Sjögren's disease (SjD) [12, 13, 14].