CD8 is a transmembrane glycoprotein and co-receptor that functions together with T cell receptor (TCR) in adaptive immune response. CD8 is mainly expressed on cytotoxic T cells as well as natural killer cells, cortical thymocytes and dendritic cells.
Furthermore, CD8 is expressed in T cell LGL leukemia and together with CD4 in some T lymphoblastic lymphoma. CD8 can be detected in colorectal carcinoma and ovarian, renal, hepatocellular and esophageal tumors.

[1] Chiba T, Ohtani H, Mizoi T et al. (2004): Intraepithelial CD8+ T-cell-count becomes a prognostic factor after a longer follow-up period in human colorectal carcinoma: possible association with suppression of micrometastasis. Br J Cancer. 91(9):1711-7.
[2] Sato E, Olson SH, Ahn J et al. (2005): Intraepithelial CD8+ tumor-infiltrating lymphocytes and a high CD8+/regulatory T cell ratio are associated with favorable prognosis in ovarian cancer. Proc Natl Acad Sci U S A. 102(51):18538-43.
[3] Gupta N, Kumar S, Singh G and Basu A (2015): Adult T cell large granular lymphocytic leukemia. Med J Armed Forces India. 71(1):272-5.
[4] Bhaker P, Das A, Rajwanshi A et al. (2015): Precursor T-lymphoblastic lymphoma: Speedy diagnosis in FNA and effusion cytology by morphology, immunochemistry, and flow cytometry. Cancer Cytopathol. 123(9):557-65.