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© Dirk Biddle
Natural killer (NK) cells are functionally similar to cytotoxic T-cells (but far more aggressive) and attack cancers and viruses. They do not need to recognize an antigen protein in a MHC molecule on the membrane of a target cell, instead they kill any cell that is not showing a specific “self” marker carried by all normal cells (such as natural cytotoxicity receptors or NCRs). NK cells kill a target cell by releasing
perforin (and other molecules) which damage the target cell membrane, leading to death by inability to maintain
osmotic balance. NK cells also cause death by inducing
apoptosis in the target. The
cytokine Tumour Necrosis Factor alpha (TNFα) is also released by the NK cells, thus NK cells may also be involved in the process of tumour destruction. Natural killer cells may also contribute to immuno-regulation by secreting high levels of influential lymphokines.
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Apoptosis: The genetically determined process of cell self-destruction that is marked by the fragmentation of nuclear DNA, is activated either by the presence of a stimulus or by the removal of a stimulus or suppressing agent, is a normal physiological process eliminating DNA-damaged, superfluous, or unwanted cells (as immune cells targeted against the self in the development of self-tolerance or larval cells in amphibians undergoing metamorphosis), and when halted (as by genetic mutation) may result in uncontrolled cell growth and tumor formation -- called also programmed cell death. (M+)
Programmed cell death as signalled by the nuclei in normally functioning human and animal cells when age or state of cell health and condition dictates.
An active process requiring metabolic activity by the dying cell, often characterised by cleavage of the DNA into fragments that give a so called laddering pattern on gels.
Cells that die by apoptosis do not usually elicit the inflammatory responses that are associated with necrosis, though the reasons are not clear.
Cancerous cells, however, are unable to experience the normal cell transduction or apoptosis-driven natural cell death process. (OMD)
Lymphokines: Any of various soluble protein factors (such as an interleukin) of low molecular weight that are not antibodies, are secreted by activated T-cells in response to stimulation by antigens, and have a primary role (such as the activation of macrophages or the enhancement or inhibition of antibody production) in cell-mediated immunity.