Viruses 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.