NK cell-mediated killing involves release of
Natural killer cells survey the surface of potential target cells for the presence of MHC class I molecules. Cells that lack or down-regulate MHC I, a common strategy of viruses and tumors, are recognized as missing-self and are killed by release of perforin and granzymes. NK cells therefore provide an important innate barrier against transformed and virally infected cells.
Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)