Plays a role in the protection against host NK cell cytotoxicity by interacting with and modulating the activity of the host inhibitory leukocyte Ig-like receptor 1/LILRB1, which is expressed on monocytes, dendritic cells, as well as subsets of T and NK cells. UL18 exerts an inhibitory effect on LIR-1+ NK cells, while it stimulates LIR-1- NK cell (By similarity). Interacts with host LILRB1.