CD38 and plasma cell myeloma: In relapsed or refractory myeloma (R/R MM) patients, the CD38 expression levels are relatively high in NK cells, regulatory B cells (Bregs), Tregs and myeloid-derived suppressor cells (MDSCs) compared with CD8+ and CD4+ T cells, indicating that CD38 is involved in providing the immunosuppressive environment for tumor growth 19.