This invention relates to NK cell activation and NK cell mediated immunity, immunogenic peptides, compositions and complexes; and associated methods of treatment or prophylaxis. In particular, a peptide capable of activating NK cell-mediated immunity, the peptide comprising or consisting of the amino acid sequence XnAX2X1 wherein Xn is an amino acid sequence of between 5 and 12 residues, and X1 is any amino acid; or leucine or isoleucine; and X2 is alanine, threonine, tryptophan, or serine.