CD19 and systemic lupus erythematosus: Before treatment with rituximab, patients with RA had significantly higher number of CD19+ cells (median 0.2260 × 109/l, range 0.012–0.663) in peripheral circulation, compared with patients with SLE (median 0.097 × 109/l, range 0.01–1.274) (Fig.  1A) (Mann–Whitney U test).