NK cells release pore-forming proteins called perforins and
proteolytic enzymes called granzymes. Granzymes pass through the
pores and activate the enzymes that lead to apoptosis, a
programmed suicide of the infected cell. Apoptosis occurs when
certain granzymes activate a group of protease enzymes called
caspases that destroy the protein structural scaffolding of the
cell, degrade the cell's nucleoprotein, and activate enzymes
that degrade the cell's DNA. As a result, the infected cell
breaks into membrane-bound fragments that are subsequently
removed by phagocytes. If very large numbers of perforins are
inserted into the plasma membrane of the infected cell, this can
result in a weakening of the membrane and lead to cell lysis
rather than apoptosis. An advantage to killing infected cells by
apoptosis is that the cell's contents, including viable virus
particles and mediators of inflammation, are not released as
they are during cell lysis.