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CD8a is the alpha chain of CD8, a transmembrane glycoprotein that is predominantly expressed on the surface of cytotoxic T cells, and can also be found on natural killer cells, cortical thymocytes, and dendritic cells. CD8 serves as a co-receptor for the T cell receptor (TCR). Both CD8 and TCR recognize antigens displayed by an antigen presenting cell (APC) in the context of class I MHC molecules. CD8 plays a role in T cell development and activation of mature T cells.