ABSTRACT
The cytolytic cascade in natural killer (NK) cell activity can be divided into four phases: binding of the effector cell to the target, triggering of the cytolytic machinery, secretion or liberation of cytolytic factors, and finally the killer cell independent phase of lysis. Some of the surface receptors involved in the binding and triggering phases have been identified, and it seems that natural killer cells recognize their targets via multiple parallel pathways instead of a single hypothetical NK cell receptor. Endogenous circulating NK cells are relatively weakly cytotoxic and rarely kill autologous uncultured tumor cells. The cytotoxicity is strongly augmented by IL-2 and also autologous malignant target cells are sensitive to IL-2-activated killer cells. The costimulation of NK-cell proliferation by IL-4 is somewhat surprising, because the inhibition of NK-cell proliferation by IL-4 has been reported. NK cells express many adhesion molecules which makes them the most adhesive lymphocyte subpopulation in peripheral blood.
