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The CD8 antigen is a cell surface glycoprotein primarily found on most cytotoxic T lymphocytes （CTLs）. It acts as a co-receptor, along with the T-cell receptor （TCR）, to recognize antigens presented by antigen-presenting cells （APCs） in the context of class I MHC molecules. The functional co-receptor can be either a homodimer composed of two alpha chains or a heterodimer composed of one alpha and one beta chain. Both alpha and beta chains share significant homology with immunoglobulin variable light chains. In research, CD8B plays a central role in the development and function of T cells. For example, it is involved in the positive and negative selection processes of T cells in the thymus, as well as the regulation of mature single-positive CD8+ T cells being released into the circulation. Moreover, the expression of the CD8B gene is finely regulated, including the regulation of CD4 and CD8A gene expression during thymic differentiation.