Integral membrane glycoprotein that plays an essential role in the immune response and serves multiple functions in responses against both external and internal offenses. In T-cells, functions primarily as a coreceptor for MHC class I molecule:peptide complex. The antigens presented by class I peptides are derived from cytosolic proteins while class II derived from extracellular proteins. Interacts simultaneously with the T-cell receptor (TCR) and the MHC class I proteins presented by antigen presenting cells (APCs). In turn, recruits the Src kinase LCK to the vicinity of the TCR-CD3 complex. A palmitoylation site in the cytoplasmic tail of CD8B chain contributes to partitioning of CD8 into the plasma membrane lipid rafts where signaling proteins are enriched. Once LCK recruited, it initiates different intracellular signaling pathways by phosphorylating various substrates ultimately leading to lymphokine production, motility, adhesion and activation of cytotoxic T-lymphocytes (CTLs). Additionally, plays a critical role in thymic selection of CD8+ T-cells. Forms disulfide-linked heterodimers with CD8A at the cell surface. Interacts with CD3D; this interaction couples TCR-CD3 with CD8. Interacts with LCK. Requires the partner CD8A for efficient cell surface expression. The heterodimer CD8A/CD8B localizes to lipid rafts due to CD8B cytoplasmic tail palmitoylation. Palmitoylated at the cytoplasmic tail and thereby targets the heterodimer CD8A/CD8B to lipid rafts unlike CD8A homodimers. The lack of Cd8b reduces but does not completely abolish thymic maturation of CD8+ T-cells. However, Cd8-depleted mice mount normal primary cytotoxic CD8 responses upon acute viral infections.