Plasma cell/blast and immunoglobulin production are low in patients with HLH. (A) Representative flow cytometry dot plot of CD38/CD19 gated on CD20⁻ cells showing depletion of the CD19⁺CD38^bright population in the HLH group (right) compared with the control group (left). (B) Percentage of plasma cell/blast (CD19⁺CD20⁻CD38^bright) population in P-HLH/I-HLH groups (n = 6) and R-HLH group (n = 4) compared with the HC (n = 8) (HC, 0.043% ± 0.006%; P-HLH/I-HLH, 0.014% ± 0.004%; and R-HLH, 0.056% ± 0.008%). (C) Representative immunohistochemistry stains of CD138 in bone marrow biopsy specimen from a patient with P-HLH (left) and 1 with R-HLH (right). (D) Percentage of CD138⁺ immunohistochemistry staining from bone marrow biopsy samples in patients with P-HLH/I-HLH (n = 10) and R-HLH (n = 6) (P-HLH/I-HLH, 0.61% ± 0.21%; R-HLH, 0.92% ± 0.41%). The two-tailed Student t test or the Mann-Whitney test was used depending on normality for statistical comparison between 2 groups. Data represent mean ± SEM; ∗P < .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001. (E-F) Peripheral blood IgG values and absolute B cells from patients with HLH before receiving steroid or immunoglobulin treatment. The open shapes represent values lower than normal range for patient age. Of the 21 patients with HLH, 19% have low IgG levels for their age. Of the 12 patients with HLH, 50% have low absolute B-cell levels for their age.