Based on cell surface markers and cell morphology, the MDSC population can be divided into granulocytic/polymorphonuclear MDSCs (G-MDSCs/PMN-MDSCs, CD33+ CD11b+ HLA-DRlowCD15+ cells) settling in the peripheral lymphoid organs, and monocytic MDSCs (M-MDSCs, CD33+ CD11b+ HLA-DRlowCD14+ cells) existing in the tumor bed (138–140). Here, CD33 is linked to neoplasm.