A fully humanized monoclonal antibody against CD123 (interleukin-3 receptor alpha chain) with potential antineoplastic activity. Anti-CD123 monoclonal antibody KHK2823 binds to and neutralizes CD123, which is upregulated on leukemic stem cells (LSC) found in myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). This agent may inhibit IL-3-dependent signaling and proliferation and may prevent the uncontrolled growth and differentiation of mutated LSC.