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.