Activated leukocyte cell adhesion molecule (ALCAM), also named as CD166 and MEMD, is a typeI transmembrane glycoprotein of immunoglobulin superfamily, which mediates homotypic and heterotypic interactions between cells. ALCAM is expressed on thymic epithelium, microvascular endothelium, activated lymphocytes and monocytes, and monocytederived dendritic cells. ALCAM mediates low-affinity adhesion with itself or the cysteine-rich scavenger receptor CD6 to regulate T cell development, immunological synapses(IS), and cell migration through endothelial junctions. ALCAM on thymic epithelia mediates adhesion to CD6 on CD4+CD8+ T cells. Adhesion of ALCAM expressing antigen presenting cells and CD6-expressing T cells stabilizes the early IS, while later it enhances CD3 effects on T cell proliferation, CD25 expression, and Th1 commitment. ALCAM may influence expression or adhesion of the neuronal adhesion molecule NCAML1, both in the developing retina and invasive melanoma.