CD8 is a transmembrane glycoprotein that serves as a co-receptor for the T-cell receptor (TCR), playing a crucial role in T cell signaling and aiding cytotoxic T cell-antigen interactions. It binds specifically to the MHC class I protein, enhancing the affinity between the T cell receptor of cytotoxic T cells and target cells during antigen-specific activation. CD8 exists in two isoforms, alpha (CD8A) and beta (CD8B), each encoded by a different gene located on chromosome 2 in humans. The active form of CD8 is a dimer, with three different dimers detected: CD8αα, CD8αβ, and CD8ββ. This co-receptor also plays a role in T cell signaling by binding Lck (lymphocyte-specific protein tyrosine kinase) via its cytoplasmic tail, initiating a cascade of phosphorylation that leads to the activation of transcription factors and affects gene expression.