The present invention concerns combinations of two or more agents for inducing an immune response to cancer or infectious disease. Agents may include leukocyte redirecting complexes, antibody-drug conjugates, interferons (preferably interferon-α), and/or checkpoint inhibitor antibodies. The leukocyte redirecting complexes have at least one binding site for a leukocyte antigen and at least one binding site for an antigen on a diseased cell or pathogen. Preferably, the complex is a DNL™ complex. More preferably, the complex comprises a bispecific antibody (bsAb). Most preferably, the bsAb is an anti-CD3× anti-CD19 bispecific antibody, although antibodies against other leukocyte antigens and/or disease-associated antigens may be used. The complex is capable of targeting effector T cells, NK cells, monocytes or neutrophils to induce leukocyte-mediated cytotoxicity of cells associated with cancer or infectious disease. The cytotoxic immune response is enhanced by co-administration of interferon, checkpoint inhibitor antibody and/or ADC.