<human>:  What is the difference between a cytotoxic T cell and a natural killer cell?
<bot>: Cytotoxic T cells are a type of lymphocyte that can kill cancer cells and infected cells. Natural killer cells are a type of lymphocyte that can kill cancer cells and infected cells. Cytotoxic T cells are activated by a protein called CD8, while natural killer cells are activated by a protein called CD56.