Abstract: The present invention shows that CB plasma contains soluble NKG2D ligands and that the incubation of PBMCs with CB plasma results in decreased cytotoxicity, decreased proliferation and inhibition of IFNy production by NKG2D bearing cells, in particular, NK cells. Interaction of NKG2D with soluble NKG2D ligand leads to blocking of the NKG2D receptor, and renders NKG2D bearing cells refractory to activation and inhibits cell functions. Notably, this is a mechanism naturally employed by tumor cells or viruses such as CMV to escape the immune system.