Flow cytometric analysis of B-cell subpopulations. PBMCs are analyzed by flow cytometry. After gating on lymphocytes according to forward (FSC)/side scatter (SSC), B cells are characterized by CD19 staining. By staining for CD27 and IgD, naive IgD⁺IgM⁺CD27⁻ B cells (1), IgD⁺IgM⁺CD27⁺ marginal zone B cells (2), and IgD⁻IgM⁻CD27⁺ switched memory B cells (3) can be distinguished. The staining for CD21 and CD38 expression allows the additional distinction of CD38^lowCD21^low B cells, (4) CD38⁺⁺IgM^high transitional B cells (5), and CD38⁺⁺⁺IgM⁻ plasmablasts (6). Examples of one healthy donor (HD) and one CVID patient (CVID) are demonstrated.