Natural killer cells can tell the difference between infected cells , tumors , and normal cells . They recognize changes in the level of a surface molecule called MHC ( major histocompatibility complex ) class I. NK cells distinguish infected cells and tumors from normal and uninfected cells by recognizing 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 .