Define Natural killer cells (NK cells)

They are large sized lymphocytes, specifically equipped to kill virally infected cells, some of the virally infected cells acquire a special glycoprotein on their surface which signals their neighbouring cells that they are in trouble and they need help.

The NK cells are especially receptive to this cry for help because they have surface proteins which are very similar to the ones which have glycoprotein coatings. When the 2 components come together in contact, the NK cells would release lethal substances which lead to the death of infected cell. The infected cell is killed and it has a chance to multiply. The major drawback of this process is that the host cell gets sacrificed.