What are cytotoxic lymphocytes and what is the purpose of their toxic granules?
Cytotoxic lymphocytes include natural killer (NK) and T (thymically-derived) killer lymphocytes. Both types of killer lymphocytes circulate in blood and through lymphoid organs of people and other mammals all the time, whether the individuals are healthy or diseased (reviewed, ref. 5). The NK and T lymphocytes control intracellular infections by viruses (with different viral species that can infect almost all cell types) or intracellular infections by species of bacteria which selectively live inside macrophages (such as tubercle bacilli). In humans, active NK lymphocytes, which lack T cell receptors for antigen, are ˜3% of all the lymphocytes circulating in blood and constitutively contain cytotoxic granules.6 NK cells are part of the innate immune system and immediately kill other cells after detecting their infection. NK cells also kill cells that are coated with specific antibodies7 which could be to viral antigens in the plasma membranes of infected cells. In contrast, memory cyto
