CD19 and systemic lupus erythematosus: Studies have reported that certain B cell subsets, including regulatory B cells (Breg, defined as CD19+CD24hiCD27+)↓, plasma cells↑, CXCR5- immature B cells↑, double-negative B cells (DNB, IgD-CD27-)↑, age-associated B cells (ABC, defined as CD11chiT-bet+)↑, unswitched memory B cells (UMBC, CD27+IgD+)↓, CD19+Siglec-10+ B cells↑, and CD226+ B cells↑ in lupus patients (74–76).