Receptor on immune cells capable to deliver stimulatory or inhibitory signals that regulate cell activation and differentiation. Exists as a GPI-anchored and as a transmembrane form, each likely initiating distinct signaling pathways via phosphoinositol 3-kinase in activated NK cells and via LCK and CD247/CD3 zeta chain in activated T cells (By similarity). Receptor for both classical and non-classical MHC class I molecules (PubMed:16177084). Receptor or ligand for TNF superfamily member TNFRSF14, participating in bidirectional cell-cell contact signaling between antigen presenting cells and lymphocytes. Upon ligation of TNFRSF14, provides stimulatory signal to NK cells enhancing IFNG production and anti-tumor immune response (PubMed:25711213). On activated CD4+ T cells, interacts with TNFRSF14 and down-regulates CD28 costimulatory signaling, restricting memory and alloantigen-specific immune response (By similarity). In the context of bacterial infection, acts as a ligand for TNFRSF14 on epithelial cells, triggering the production of antimicrobial proteins and pro-inflammatory cytokines (PubMed:22801499). The soluble GPI-cleaved form, usually released by activated lymphocytes, might play an immune regulatory role by limiting lymphocyte effector functions. Homomultimer; disulfide-linked (By similarity). Interacts with classical and non-classical MHC class I molecules (PubMed:16177084). Interacts with TNFRSF14 (via cysteine-rich domain 1); this interaction is direct (PubMed:18193050). Interacts with LCK and CD247/CD3 zeta chain (By similarity). Released from the cell membrane by GPI cleavage. Expressed in resting and activated NK cell subsets (at protein level) (PubMed:25711213, PubMed:16177084). Expressed in resting NKT cells (at protein level) (PubMed:16177084). Expressed in activated CD8+ T cells (at protein level). Highly expressed in intraepithelial lymphocyte (IEL) subsets, particularly in innate-like CD8A-positive IELs (at protein level) (PubMed:22801499). No visible phenotype.