Purity of primary human B-cell preparations. B cells were isolated from PBMCs by CD19 positive selection. Purity was assessed by flow cytometry (see “Materials and methods”) using CD22, CD3, CD16, and CD14 staining to detect B cells, T cells, NK cells, and monocytes, respectively. The percentage of stained cells is indicated in each panel. Dotted lines correspond to isotypic controls.