All armies have specialists who are expert in specific kinds of combat.
NK cells lyse, or burst, target cells after binding to them and damaging their cell membranes. NK cells do not kill indiscriminately, however. They save their wrath for cancerous cells, virus-infected cells, and other interlopers, while normal cells are spared. If MHC class I molecules are absent on the target cell, or expressed in reduced amounts, the NK cells proceed with their attack. I molecules trigger CTLs to kill the target cell.