We use microchip techniques for live cell imaging and screening to characterize immune response at the single cell level. T cell and natural killer (NK) cells have remarkable abilities to mediate targeted attack against virus-infected or malignant cells. In adoptive cell therapy these qualities are used to treat primary malignancies, life-threatening virus infections or leukemic relapses after allogeneic hematopoietic stem cell transplantation. Although this therapy has shown promising results we have only a remote understanding of how to utilize T and NK cells to their full potential. A major challenge is to select and prepare cells that mediate anti-tumor effects without causing life-threatening side effects such as graft versus host disease. By better understanding of how populations of NK cells and T cells are built up by individual cells with varying capacity to respond to stimuli we will be able to improve current methods for immunotherapy.