CD8 (cluster of differentiation 8) is a transmembrane glycoprotein that functions as a co-receptor with the T-cell receptor (TCR). The CD8 molecule consist of a heterodimer of α and β chains covalently linked by a disulfide bond, and are predominantly expressed on the surface of cytotoxic T cells. CD8 expression can also be detected on natural killer cells, cortical thymocytes, and dendritic cells. Cytotoxic CD8+ T lymphocytes are crucial components of the adaptive immune system that execute immunosurveillance to eliminate nascent tumor cells. Upon simultaneous binding to the major histocompatibility complex (MHC) class I molecule with TCR, cytotoxic T-cell sensitivity is increased 100-fold. CD8 is commonly expressed in T-cell large granular lymphocyte leukemia, and is also co-expressed with CD4 in some T-lymphoblastic lymphoma. Recently, detection of tumor infiltrating CD8+ lymphocytes have been correlated with favorable prognosis and improved survival in patients with colorectal, ovarian, esophageal, renal, lung and pancreatic tumors. A high CD8+/CD4+ T-cell ratio is also associated with improved survival in colon and ovarian cancer patients.