Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective treatment for hematological malignancies, with graft-versus-leukemia (GVL) effects playing a crucial role in disease eradication. This article reviews the principles of cellular immunotherapy, focusing on exploiting alloreactivity to induce anti-leukemic responses while minimizing prolonged donor cell persistence and associated graft-versus-host disease (GVHD). We discuss the mechanisms of alloreactivity, the clinical evidence supporting this approach, and future directions for optimizing cellular immunotherapy strategies.