NK cells are the cells in the body that fight disease and infection. NK cells may destroy tumor cells, particularly when the NK cells are "mismatched" for certain proteins, called human leukocyte antigens (HLA). Researchers think that this ability to destroy tumor cells can be predicted by learning the donor's and the recipient's HLA types to see if they are a mismatch, and by checking for other specialized proteins found on the surface of the donor's NK cells. These proteins are called killer immunoglobulin receptors (KIR).