CD8 (or Cluster of Differentiation 8) is a T-cell co-receptor. CD8 binds to class I MHC protein. CD8 exists as 2 isoforms: α and β. CD8 forms a dimer of CD8α and CD8β. The extracellular domain of CD8 has immunoglobulin variable-like conformation. This domain of CD8α interacts with α3 domain of class I MHC enhancing the binding of the T-cell receptor and the target cell.