The human BM is the major site of PCs lacking expression of CD19. (A) CD19 expression by PB/PC (black histograms) was analyzed by flow cytometry in peripheral blood, tonsil, lymph node (LN), BM, and spleen samples (supplemental Figure 1). CD19 expression by total lymphocytes (dashed lines) and isotype control-stained PC (shaded) are shown for comparison in representative histograms. Horizontal bars indicate gates used to determine frequencies of PC lacking CD19. Frequencies of CD19⁻ PC among total PC were determined (B, median values are indicated from 21 blood samples, 6 spleen samples, 1 LN/7 tonsil samples, and 34 femoral head BM samples), and numbers of CD19⁻ and CD19⁺ PC per organ were extrapolated for 21 blood samples, 6 spleen samples, 1 LN/7 tonsil samples, and 25 femoral head BM based on²⁰  (C, median values and interquartile range shown). Significance levels (***P < .001) obtained from Mann-Whitney U tests are shown for comparisons between BM data and other tissues. The grey data point in (B) indicates a parotid LN. (D) Percentages of PC lacking CD19 expression were analyzed in BM samples from iliac crest (36 samples), femoral head (34 samples), and sternum (9 samples). Kruskal-Wallis testing was used for statistical analysis. Consistent frequencies of CD19⁻ PC were determined using various anti-CD19 clones (supplemental Figure 1F).