**7. "Missing self" hypothesis**

NK cells lack antigen-specific receptors, but their activation can be blocked by an inhibitory signal generated by their recognition of MHC I alleles on host cells. However, the absence of MHC molecules I triggers an activating signal [19]. Nevertheless, the recognition of MHC I molecules would be one of the major causes of the tumor escape from NK cell immune surveillance and activation. Therefore, among the current therapeutic strategies is the use of monoclonal antibodies that target the NK cell inhibitory receptors.
