Illustration of cytotoxic T cells (blue) attacking cancer cells (red). T cells, or T lymphocytes, are a type of white blood cell that recognise a specific site (antigen) on the surface of cancer cells or pathogens and bind to it. Some T lymphocytes then signal for other immune system cells to eliminate the cell. Cytotoxic T lymphocytes eliminate the cell themselves. They attach to the target cell and secrete the protein perforin, which pierces the cell membrane, allowing cytotoxin granules to enter the cell. The cytotoxin granules expel granzymes which promote cell apoptosis (programmed cell death).