The CD8 antigen is a cell surface glycoprotein found on most cytotoxic T lymphocytes that serves as a coreceptor for TCR recognition of MHC class I–associated peptides and supports CTL activation by binding to the MHC, while making no direct contact with the peptide.CD8 is expressed on cytotoxic suppressor T cells. It is expressed as a heterodimer of CD8α and CD8β. CD8 is expressed on approximately one-third of peripheral blood T cells (the CD4-negative cells). CD8 has also been detected at a low level on some natural killer (NK) cells. In normal human tonsil, large numbers of CD8+ lymphocytes were present within the paracortex; occasionally positive cells were also identified within germinal centers and within the investing squamous epithelium. In other tissues, only lymphoid cells and cells of histiocyte lineage showed positive staining for CD8.