The paired immunoglobulin-like type 2 receptors (PILR) comprise the inhibitory receptor PILRA and the activating receptor PILRB (Shiratori et al. 2004). The inhibitory PILRA is mainly expressed on macrophages, dendritic cells and granulocytes, whereas the activating PILRB is mainly on activated NK cells. Both recognize mouse CD99 as a ligand, but the binding affinity of PILRB is much lower (Tabata et al. 2008). Mouse NK cells expressing PILRB mediate cytotoxicity against CD99-positive target cells, suggesting that this receptor may be involved in NK cell recognition (Shiratori et al. 2004).