Cell adhesion molecule that mediates both heterotypic cell-cell contacts via its interaction with CD6, as well as homotypic cell-cell contacts. Promotes T-cell activation and proliferation via its interactions with CD6 (By similarity). Contributes to the formation and maturation of the immunological synapse via its interactions with CD6 (By similarity). Mediates homotypic interactions with cells that express ALCAM. Mediates attachment of dendritic cells onto endothelial cells via homotypic interaction. Inhibits endothelial cell migration and promotes endothelial tube formation via homotypic interactions. Required for normal organization of the lymph vessel network. Required for normal hematopoietic stem cell engraftment in the bone marrow. Plays a role in hematopoiesis; required for normal numbers of hematopoietic stem cells in bone marrow. Promotes in vitro osteoblast proliferation and differentiation (By similarity). Promotes neurite extension, axon growth and axon guidance; axons grow preferentially on surfaces that contain ALCAM (By similarity). Mediates outgrowth and pathfinding for retinal ganglion cell axons (By similarity). Homodimer. Interacts (via extracellular domain) with CD6 (via extracellular domain). Homodimerization and interaction with CD6 involve the same region and cannot occur simultaneously. The affinity for CD6 is much higher than the affinity for self-association. Interacts (via glycosylated extracellular domain) with LGALS1 and LGALS3. Interaction with LGALS1 or LGALS3 inhibits interaction with CD6. Detected at the immunological synapse, i.e, at the contact zone between antigen-presenting dendritic cells and T-cells. Colocalizes with CD6 and the TCR/CD3 complex at the immunological synapse. Constitutively expressed in the autonomic nervous system. Sympathetic and parasympathetic nerve fibers but not myelinated nerve fibers in the spinal nerve. The CD6 binding site is located in the N-terminal Ig-like domain. Glycosylated.