CD38 expression in various cell populations. Histograms show expression of CD38 in T cells, B cells, monocytes, and NK cells (A) in a healthy donor and (B) in a patient with MM. CD38 expression in plasma cells from the myeloma patient is also shown. (C) Subpopulations of NK cells, Tregs, Bregs, and MDSCs from relapsed and refractory myeloma patients expressed higher levels of CD38 compared with CD8⁺ and CD4⁺ T cells. MFI, mean fluorescent intensity.