Cytotoxic T lymphocytes (CTL) recognize surface markers on other cells in the body that label those cells for destruction. In this way, CTLs help to keep virus-infected or malignant cells in check. In the video above, the smaller CTL is attacking and killing a much larger influenza virus-infected target. The sequence represents 30 minutes elapsed time. The changing morphology of the target cell is typical of that seen in the process of apoptosis.