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 .