It’s a lock and key system. T-cells are incredibly diverse individuals (due to evolution) in what antigens they can be triggered for. When a Dendritic Cell initially expresses to t-cells, most t-cells will move on until the right t-cell locks in to the right antigen on a DC expressing many antigens. (This happens in the lymph nodes). Once the right t-cell locks up with with dendritic cell’s antigen, the dendritic cell then transfers co-stimulatory molecules and cytokine stimulation to tell the t-cell how much to multiply and how aggressive to be toward the target tumor cells.