Original: NK cells distinguish infected cells and tumors from normal and uninfected cells by recognizing level changes of a surface molecule called MHC ( major histocompatibility complex ) class I. NK cells are activated in response to a family of cytokines called interferons .
Simplified: They recognize changes in the level of a surface molecule called MHC ( major histocompatibility complex ) class I. NK cells are activated by interferons released by damaged cells .