TNF and neoplasm: We show that CD3xHER2 bsAb‐mediated interaction of T‐cells with a HER2+ tumor, triggers (1) bystander killing of up to 50% HER2− cells in mixed areas and (2) IFNγ/TNFα‐mediated paracrine killing that reaches HER2− tumor areas up to ∼250 μm distance in a 3D co‐culture setup.