Function: One of the major glycoproteins of thymocytes and T lymphocytes. Plays a role in the physicochemical properties of the T-cell surface and in lectin binding. Presents carbohydrate ligands to selectins. Has an extended rodlike structure that could protrude above the glycocalyx of the cell and allow multiple glycan chains to be accessible for binding. Is a counter receptor for SN/Siglec-1 (By similarity). During T-cell activation is actively removed from the T-cell-APC (antigen-presenting cell) contact site thus suggesting a negative regulatory role in adaptive immune response. Tissue specificity: Cell surface of thymocytes, T-lymphocytes, neutrophils, plasma cells and myelomas. PTM: Glycosylated; has a high content of sialic acid and O-linked carbohydrate structures.