Cell adhesion molecule that mediates cell-cell contacts and regulates T-cell responses via its interaction with ALCAM/CD166 (PubMed:15048703, PubMed:15294938, PubMed:16352806, PubMed:16914752, PubMed:24945728, PubMed:24584089). Contributes to signaling cascades triggered by activation of the TCR/CD3 complex (PubMed:24584089). Functions as costimulatory molecule; promotes T-cell activation and proliferation (PubMed:15294938, PubMed:16352806, PubMed:16914752). Contributes to the formation and maturation of the immunological synapse (PubMed:15294938, PubMed:16352806). 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 (PubMed:17601777). LPS binding leads to the activation of signaling cascades and down-stream MAP kinases (PubMed:17601777). Mediates activation of the inflammatory response and the secretion of pro-inflammatory cytokines in response to LPS (PubMed:17601777). Interacts (via extracellular domain) with ALCAM/CD166 (via extracellular domain) (PubMed:15048703, PubMed:16352806, PubMed:16914752, PubMed:24945728, PubMed:26146185). Interacts with the TCR/CD3 complex subunit CD3E (PubMed:15294938). Interacts (via tyrosine phosphorylated C-terminus) with LCP2 (via SH2 domain) (PubMed:16914752). Interacts with VAV1 (By similarity). Interacts (via glycosylated extracellular domain) with LGALS1 and LGALS3 (PubMed:24945728). Interaction with LGALS1 or LGALS3 inhibits interaction with ALCAM (PubMed:24945728). Detected at the immunological synapse, i.e, at the contact zone between antigen-presenting dendritic cells and T-cells (PubMed:15294938, PubMed:16352806). Colocalizes with the TCR/CD3 complex at the immunological synapse (PubMed:15294938). The origins of the secreted form are not clear, but it might be created by proteolytic shedding of the ectodomain. Detected on thymocytes (PubMed:15294938). Detected on peripheral blood T-cells (PubMed:15048703, PubMed:16352806). Detected on natural killer (NK) cells (PubMed:16352806). Soluble CD6 is detected in blood serum (at protein level) (PubMed:17601777). Detected in spleen, thymus, appendix, lymph node and peripheral blood leukocytes (PubMed:9013954). Expressed by thymocytes, mature T-cells, a subset of B-cells known as B-1 cells, and by some cells in the brain. After T-cell activation, becomes hyperphosphorylated on Ser and Thr residues and phosphorylated on Tyr residues. Glycosylated. Lacks the third SRCR domain and doesn't bind ALCAM/CD166. Doesn't localize to the immunological synapse. Lacks the third SRCR domain and doesn't bind ALCAM/CD166. Doesn't localize to the immunological synapse. Constitutes the only expressed species in a small percentage of T-cells.