The CD8 antigen (Cluster of Differentiation 8) is a cell surface glycoprotein consisting of two subunits, alpha and beta. Anti-CD8 is a T-cell marker for the detection of cytotoxic/suppressor lymphocytes. CD8 is also detected on NK cells, some thymocytes, some null cells and bone marrow cells. This antibody can be used together with other markers to differentiate between reactive and neoplastic T cells. CD8 expression has been found to be negative in mycosis fungoides. Rarely, anti-CD8 marks non-hematolymphoid neoplasms.