Plasma cells in human bone marrow. (A) Vaccinia-specific IgG-secreting cells (top) and total IgG-secreting cells (bottom) of total BM mononuclear cells compared with control are shown (50 000 cells per well were seeded). The cells are from the same patient. The plot shows the frequency of vaccinia-specific IgG-secreting cells among total BM mononuclear cells of all patients examined (n = 26). (B) Gating strategy to sort plasma cells into CD19⁺ (left) and CD19^– populations (right). (C) The percentage of plasma cells of 100 000 live BM mononuclear cells (left) and the percentage of CD19⁺ and CD19^– plasma cells of total plasma cells (right), gated as in panel B, are shown (n = 27). (D) CD19⁺ and CD19^– plasma cells secreting vaccinia-specific antibodies (top) with matching wells showing the secretion of total IgG (bottom). All wells are from the same patient and 5000 cells of each type were seeded. The plot shows paired CD19⁺ and CD19^– vaccinia-specific IgG⁺ plasma cells from the same patient (n = 19). *P < .05; **P < .01; ***P < .001.