Immune receptor involved in self-nonself discrimination. In complex with KLRC1 or KLRC2 on cytotoxic and regulatory lymphocyte subsets, recognizes non-classical major histocompatibility (MHC) class Ib molecule HLA-E loaded with self-peptides derived from the signal sequence of classical MHC class Ia and non-classical MHC class Ib molecules, .Enables cytotoxic cells to monitor the expression of MHC class I molecules in healthy cells and to tolerate self. Primarily functions as a ligand binding subunit as it lacks the capacity to signal.