Cell adhesion molecule that mediates cell-cell contacts and regulates T-cell responses via its interaction with ALCAM/CD166. Contributes to signaling cascades triggered by activation of the TCR/CD3 complex (PubMed:24584089). Functions as costimulatory molecule; promotes T-cell activation and proliferation. Contributes to the formation and maturation of the immunological synapse. Functions as calcium-dependent pattern receptor that binds and aggregates both Gram-positive and Gram-negative bacteria. Binds both lipopolysaccharide (LPS) from Gram-negative bacteria and lipoteichoic acid from Gram-positive bacteria. LPS binding leads to the activation of signaling cascades and down-stream MAP kinases. Mediates activation of the inflammatory response and the secretion of pro-inflammatory cytokines in response to LPS. Interacts (via extracellular domain) with ALCAM/CD166 (via extracellular domain) (PubMed:16914752). Interacts with the TCR/CD3 complex subunit CD3E. Interacts (via tyrosine phosphorylated C-terminus) with LCP2 (via SH2 domain) (PubMed:24584089). Interacts (via glycosylated extracellular domain) with LGALS1 and LGALS3. Interaction with LGALS1 or LGALS3 inhibits interaction with ALCAM (By similarity). Interacts with VAV1 (PubMed:24584089). Detected at the immunological synapse, i.e, at the contact zone between antigen-presenting dendritic cells and T-cells. Colocalizes with the TCR/CD3 complex at the immunological synapse. Additional isoforms seem to exist. Expressed predominantly in thymus, lymph node and spleen. After T-cell activation, becomes hyperphosphorylated on Ser and Thr residues (By similarity). Phosphorylated on tyrosine residues in response to stimulation of the TCR complex (PubMed:24584089). Glycosylated.