IgM surface expression and light chain usage in CD21^low B cells. (A) Representative FACS plots for the expression of indicated B-cell markers in B cells of CVID patients (n = 10). CD19⁺IgM⁺ B cells were gated on CD21^low and CD21⁺ naive B cells, according to CD38 and CD21 expression, and IgM surface expression was analyzed as the mean fluorescence intensity (MFI) in CD21⁺ and CD21^low B-cell subpopulations, respectively. (B) The diagram shows no significant difference in the cell-surface expression of IgM between CD21⁺ and CD21^low B cells in CVID. (C) Representative FACS plots demonstrate the expression of IgM and λ light chain on CD21^low and naive B cells of CVID patients (n = 4). (D) The diagram shows a significantly decreased usage of κ-light chains in CD21^low B cells of CVID patients compared with naive B cells of the same individuals.