CD8

CD8 (cluster of differentiation 8) is a transmembrane glycoprotein which serves as a co-receptor for the T cell receptor (TCR). Like the TCR, CD8 binds to a major histocompatibility complex (MHC) molecule, but is specific for the class I MHC protein. It is predominantly expressed on the surface of cytotoxic T cells, but can also be found on natural killer cells. To function, CD8 forms a dimer, consisting of a pair of CD8 chains. The most common form of CD8 is composed of an CD8-α and CD8-β chain, both members of the immunoglobulin superfamily with an immunoglobulin variable (IgV)-like extracellular domain connected to the membrane by a thin stalk, and an intracellular tail. Less common homodimers of the CD8-α chain are also expressed on some cells. The extracellular IgV-like domain of CD8-α interacts with to the α3 portion of the Class I MHC molecule. This affinity keeps the T cell receptor of the cytotoxic T cell and the target cell bound closely together during antigen-specific activation. Cytotoxic T cells with CD8 surface protein are called CD8+ T cells.