T-cell differentiation antigen CD6 is a type 1 transmembrane cell adhesion molecule that is a member of the scavenger receptor cysteine rich (SRCR) family of proteins.? CD6 is composed of three scavenger receptor cysteine-rich (SRCR) extracellular domains, a transmembrane region, and a long cytoplasmic tail without intrinsic catalytic activity.? It is expressed on peripheral blood T lymphocytes, thymocytes, subset of NK and B cells, and some hematopoietic cell precursors and brain cells.? Soluble form of CD6 is detected in blood serum.?? CD6 has two ligands, CD166 (ALCAM) and CD318. It plays an important role in the stabilization and maturation of immunological synapses by interacting with ALCAM.? Domain 3 of CD6 binds to ALCAM via the N-terminal domain while CD318 interacts with domain 1 of CD6. CD6 plays a role in cell to cell contacts, regulating T-cell responses via CD6-ALCAM interaction, as well as functioning as a costimulatory molecule promoting T-cell activation and T-cell proliferation by its involvement in the TCR/CD3 signaling cascade complex.? CD6 has been shown to have a high binding affinity to highly conserved bacterial components such as lipopolylsaccharides (LPS) and Lipoteichoic acid (LTA), which allows it to help regulate inflammatory responses and secretion of cytokines in response to bacterial toxins.? CD6 is also a popular target for the development of new therapeutic methods for autoimmune diseases such as multiple sclerosis due to its involvement in T-cell proliferation and activation.