Natural killer cells

Natural killer (NK) cells are a part of the innate immune system. This subset of lymphocytes was first identified in the 1970s as a set of cells that were neither B or T and that could kill target cells without prior sensitization (Greenberg, 1994). The term natural killerwas devised by Kiessling and co-workers in 1975 (Kiessling, Klein & Wigzell, 1975) to describe these naturally occurring lymphocytes with lytic activity against syngeneic or allogenic cells.
NK cells descend from the same lineage as T cells, thus sharing some of the characteristics of these lymphocytes, such as cytolytic activity and lymphokine (lymphocyte-produced cytokines) production. However, NK cells do not undergo T cell receptor gene rearrangement to create diverse receptors as part of the adaptive immune system (Caligiuri, 2008; Moretta, Ciccone, Mingari, Biassoni & Moretta, 1994). Therefore, the mechanisms through which NK cells lyse tumour and virus-infected cells are regarded as non-specific (Trinchieri, 1989). Recent studies have related altered levels of circulating NK cells and/or activity with cancer development, acute viral infections and autoimmune diseases, as summarized in Table 1 .