QXL138AM binds CD138, not CD64. After CD138 binding, tumor proteases unmask IFN-α to induce apoptosis/antiproliferative signaling in CD138+ tumor cells and activate immunity. The IgG1 Fc engages Fcγ receptors (including CD64) on immune cells to mediate ADCC/CDC against CD138+ targets. CD64+ cells serve as effectors and are not directly targeted or killed.