CD27⁺ B cells are decreased in X-HIgM. PBMCs from patients with X-HIgM and NDs were stained with CD19, IgD, and CD27 fluorochrome-conjugated antibodies and analyzed by flow cytometry using CELLQuest software. The percentage of naive (CD19⁺IgD⁺CD27⁻), preswitch (CD19⁺IgD⁺CD27⁺), and postswitch (CD19⁺IgD⁻CD27⁺) cells represent the data analyzed from four 7- to 13-year-old patients with X-HIgM and 4 NDs. *Significant (P < .001) difference in the percentage of cells between ND and X-HIgM.