are distinguished from cells such as macrophages that eat other cells or munch debris by a method called phagocytosis. Killers do not use phagocytosis, they just kill and leave the clean-up to other cells. Killers are known to attack virus-infected cells and cells that have become cancerous. Because of these abilities much research has been done into transforming these qualities into medical therapy but progress has been slow. Here is the parade of killers and how they work: Cytotoxic T cells (also called Tc or antigen-specific cyto -lytic or -toxic T lymphocytes (CTL)). Tc kill by apoptosis and either splash their target with perforin or granzymes or else use Fas-Fasl Interaction to command target elimination. This kills cells that are infected and display antigen. NK cells (also called LGL (large granular lymphocytes)) These kill with exactly the same methods as Tc, but have no interaction with any antigen. They select their targets based on typical molecules displayed by cells that are under stress by viral infection. NK Cells mainly are in the circulation (5-15% of the circulating lymphocytes) yet are also distributed in tissues everywhere. LAK cells (Lymphokine-activated killer) are a laboratory/clinical subset of NK Cells promoted by IL-2 to attack tumor cells. NKT cells see Natural Killer T cell main article Natural Killer T Cells. Human NK T cells are a unique population (which express NK cell markers such as CD56 and KIR). NKT cells are thought to play an important role in tumor immunity and immunoregulation. (Medical Immunology, p. 135), yet little is known. Recent evidence suggests a role working together with hepatic stellate cells being a liver-resident antigen-presenting cell that presents lipid antigens to and stimulates proliferation of NKT cells. Natural killer-like T cells A heterogeneous group with ill-defined properties. However, in summary there is no known cell or set of cells that is capable of killing cancerous cells in general. Labeling lymphopoiesis Because all WBCs are microscopic, colorless and often seemingly identical in appearance they are individually identified by their natural chemical markers, many of which have been analyzed and named. When two cells have the same markers, the reasonable assumption is made that the cells are identical at that time. A set of markers is colloquially described as the barcode for that cell or that cell line. Here is an example of how a barcode can come to be, for the all-important hematopoietic stem cell