How do natural killer cells function?

Two kinds of cell surface receptors are presented on natural killer cells (NK cells), inhibitory and activating receptors, which are differentiated based on function. NK cell activation is determined by the balance of inhibitory and activating receptor stimulation. Normal target cells are protected from killing by NK cells when signals delivered by stimulatory ligands are balanced by signals delivered by inhibitory ligands like self MHC class I molecules; whereas cells with abnormalities, such as loss of MHC class I expression or the elevated expression of specific stress-induced ligands, would strike the balance and trigger NK cells into cell killing process.

Once activated, NK cells can eliminate virus-infected or malignantly transformed cells through three categories of functions: 1) Cytotoxicity: With large numbers of cytolytic granules containing proteins such as perforin and various granzymes, NK cells can kill certain virally infected cells and tumor target cells by releasing these proteins to a cell slated for killing and inducing either apoptosis or osmotic cell lysis; 2) Cytokine and chemokine secretion: NK cells are well noted for the ability to produce a number of cytokines and chemokines; 3) Contact-dependent cell co-stimulation: NK cells can express several co-stimulatory ligands and provide co-stimulatory signals to T cells or B cells, serving as a bridge in an interactive loop between innate and adaptive immunity.