They may sound sinister, but natural killer cells are a type of white blood cell that’s a vital member of the immune system’s rapid-response team against virus-infected and cancerous cells. They’re called natural killers because they don’t require prior exposure to a disease to be activated, unlike some other parts of the immune system that can only respond effectively after a history of exposure, as in the case of, say, chicken pox.41 After all, you don’t want to wait until your second tumor appears before your immune system starts fighting.
