Natural killer (NK) cells are a population of lymphocytes that function in both immune
defense and reproduction. Diversifying NK cell phenotype and function are interactions
between NK cell receptors and major histocompatibility complex (MHC) class I ligands. As a
consequence of strong and variable selection these ligand–receptor systems are
polymorphic, rapidly evolving, and considerably species-specific. Counterparts to the human
system of HLA class I ligands and killer cell immunoglobulin-like receptors (KIR) are present …