text: Paired receptors transduce extracellular signals through opposing intracellular signaling pathways. Canonically, inhibitory receptors recruit phosphatases through their ITIM motifs, inhibiting the function of cells in which they are expressed. By contrast, activating receptors interact with adaptor proteins such as DAP-12 bearing an ITAM motif, which in turn recruit kinases such as Syk and ZAP70. 
  Ligands for paired receptors can be very diverse. They are often proteins; the best-characterized are the MHC class I molecules, but a number of other endogenous molecules have been described as ligands for at least one family of paired receptors, and in a few cases in the LILR family, even intact bacteria or viruses can serve as ligands. Lipids such as phosphatidylethanolamine and phosphatidylserine, sugars and sialylated glycans, and nucleic acids can all serve as ligands for some paired receptors.
  The binding affinity of paired receptors' extracellular domains for their ligands is generally fairly weak, with dissociation constants (K) in the micromolar (μM) range. However, the inhibitory member of a pair usually binds with higher affinity than the activating member. This can produce a competitive inhibition effect, in which the inhibitory member of the pair out-competes its activating counterpart for ligand binding; other mechanisms of interference with activation, such as disrupting dimerization, have also been described. Thus the net baseline signal from the pair is usually inhibitory, but may be modulated through differences in expression, surface density, subcellular localization, or other factors. 
  In NK cells, ligands for inhibitory receptors are often MHC class I (MHC-I) molecules, while those for activating receptors  may include signals of abnormality or infection such as proteins from pathogens or tumors, or molecules associated with cell stress. Endogenous ligands for inhibitory receptors are better characterized than those for activating receptors. Paired receptor signaling may represent maintenance of homeostasis such that immune responses to normal host cells are inhibited, while responses to abnormal or pathogenic molecules in the environment are activating.  NK activation in the absence of inhibitory receptor signals from endogenous ligands is a molecular mechanism for the missing-self hypothesis of NK activation.