CD8+ (cytotoxic) T cells, like CD4+

CD8+ (cytotoxic) T cells, like CD4+ Helper T cells, are generated in the thymus and express the T-cell receptor. However, rather than the CD4 molecule, cytotoxic T cells express a dimeric co-receptor, CD8, usually composed of one CD8α and one CD8β chain. CD8+ T cells recognise peptides presented by MHC Class I molecules, found on all nucleated cells. The CD8 heterodimer binds to a conserved portion (the α3 region) of MHC Class I during T cell/antigen presenting cell interactions (see Figure 1).CD8+ T cells (often called cytotoxic T lymphocytes, or CTLs) are very important for immune defence against intracellular pathogens, including viruses and bacteria, and for tumour surveillance. When a CD8+ T cell recognises its antigen and becomes activated, it has three major mechanisms to kill infected or malignant cells. The first is secretion of cytokines, primarily TNF-α and IFN-γ, which have anti-tumour and anti-viral microbial effects.