Interaction between the human NK receptors and their ligands

NK cells are physiologically important in a number of contexts: mediating i mmunity against viruses, intracellular bacteria and parasites, and in anti- tumour immune responses. Moreover, in addition to these overtly immune prot ective functions, NK cells also mediate a variety of homeostatic functions, particularly in the regulation of haematopoesis and they may have an impor tant role to play in the maintenance and development of placentation; certa inly NK cells are a major component of the lymphocyte population of the dec idua. The behaviour of the NK cell in these various situations is regulated by a large number of distinct receptors that transmit positive and negativ e signals. The balance of these signals determines whether the NK cell does nothing or is activated to proliferate, kill or secrete a wide range of cy tokines and chemokines. Ln this review the structure and function of a numb er of molecules found on the NK cell surface are discussed, particular emph asis being placed on the molecular details of the recognition of target cel l classical class I HLA molecules by Killer cell Immunoglobulin-like Recept ors (KIR) and the binding of the non-classical class I molecule HLA-E to th e heterodimer formed by the association of CD94 with various members of the NKG2 proteins.