Natural killer cells (or NK cells) are a type of cytotoxic (or cell toxic) lymphocyte that constitutes a major component of the immune system.

NK cells play a major role in the killing of cancerous tumors and cells infected by viruses. The cells kill by releasing small granules of proteins called perforin and granzyme that cause the target cell to die by apoptosis (cell suicide) or necrosis.

References

Cellular and Molecular Immunology by Abbul K. Abbas & Andrew Lichtman Saunders Copyright 2003