OTHER FUNCTIONAL CELLS
NK cells also can kill target cells in the absence of either antigen or antibody stimulation. Their lineage is not known, but they are probably in some manner related to T cells. Unlike other cells, they can be nonspecifically activated by mitogens, interferons, and IL-12. These cells are particularly useful in the early response to viral infection. As in other cells, they have receptors on their surface that recognize particular ligands. For example, NKR-PI is a lectin-like receptor that recognizes carbohydrate moieties on target cells, which initiates killing. As in other cell systems, there is also an inhibiting receptor called KIR. This molecule binds to ligands on MHC class I ligands, and this prevents killing of the target cells.