When cells are either under stress, are turning into tumors, or are
infected, various stress-induced molecules are produced and are put
on the surface of that cell. In addition, v iruses and malignant
transformation can sometimes interfere with the ability of the infected cell
or tumor cell to express MHC-I molecules. Without the signal from the killer-inhibitory
receptor, the kill signal from the killer-activating signal is not overridden
and the NK cell
releases pore-forming proteins called perforins, proteolytic enzymes called
granzymes, and chemokines. Granzymes
pass through the pores and activate the enzymes that lead to apoptosis of
the infected cell by means of destruction
of its structural cytoskeleton proteins and by chromosomal
degradation. As a result, the
cell breaks into fragments that are subsequently removed by phagocytes. Perforins
can also sometimes result in cell lysis.
