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: after they mature , the lymphocytes enter the circulation and lymphoid organs ( e.g. the spleen and lymph nodes ) where they are able to sense invading pathogens and\/or tumor cells .