The CD8αβ heterodimer is a key cell surface co-receptor on cytotoxic T lymphocytes. It binds to MHC class I molecules on antigen-presenting cells, stabilizing the interaction between the T-cell receptor (TCR) and the peptide-MHC complex. This binding is essential for T-cell activation, positive selection in the thymus, and the effector function of mature CD8+ T cells. The β-chain (CD8β) modulates the affinity of the interaction, contributes to signal transduction, and promotes the stable surface expression of the heterodimer.