Natural killer cells are a type of cell in the immune system. They're not disease specific and react very well to viruses and tumors. Unlike other cells in the immune system that react to antibodies or to foreign cells, natural killer cells react to cell stress and can therefor activate much more quickly, usually within three days of an infection.
They work by releasing a toxin near the target cell which penetrates the cell wall, resulting in its destruction.
However, natural killer cells also play a part in suppressing the immune system, particularly during pregnancy.
They are currently being studied as a possible treatment for cancer and AIDS.