CD19 and systemic lupus erythematosus: More importantly, both soluble BCMA (r = 0.24, p = 0.029) and surface BCMA on B cells (r = 0.55, p < 0.0001) and plasmablasts (r = 0.46, p < 0.0001) strongly correlated with the frequency of circulating plasmablasts among CD19+ B cells, identifying these markers as surrogates for humoral activity in SLE.