Loss of CD19⁺ memory B lymphocytes in XLP patients. Dot plots correspond to individual flow cytometry studies of PBMC samples (from XLP patients 9 and 4 and from 1 healthy control subject, N) stained with anti-CD27 phycoerythrin (PE) and anti-CD19 fluorescein isothiocyanate (FITC) monoclonal antibodies. The percentage of memory CD19⁺ lymphocytes that expressed CD27 (CD27⁺CD19⁺/CD19⁺) was calculated for XLP patients 9 and 4 at different times. Shaded area in the graph corresponds to mean ± SEM of healthy control subjects performed simultaneously (n = 7).