BCR and systemic lupus erythematosus: It has been proven that XmAb5871 causes phosphorylation of the ITIM of FcgRIIb and inhibits BCR-induced calcium mobilization, proliferation, and costimulatory expression of human B cells from both healthy donors and systemic lupus erythematosus (SLE) cases (29), and also B cell proliferation stimulated by lipopolysaccharides (LPS), interleukin 4 (IL-4), or B-cell activating factor (BAFF).