CD8 alpha subunit <p>The CD8 alpha subunit is a component of the CD8 alpha-beta heterodimer and is additionally found as a homodimeric complex [<cite idref="PUB00034830"/>]. Both of these species assist in the recognition of MHC class I-peptide complexes by the alpha-beta T cell receptor (TCR). CD8 binds to constant regions of the peptide-MHC complex and augments antigen-specific interactions that are formed by the TCR [<cite idref="PUB00034831"/>]. Additionally, CD8 is directly coupled to TCR signalling by virtue of its interactions with the p56Lck kinase [<cite idref="PUB00034832"/>], as well as other proteins. CD8 expression is limited to and one of the defining characteristics of cytolytic T cells.</p>