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CD223, also known as LAG-3 （lymphocyte activation gene-3）, is a type I transmembrane protein belonging to the immunoglobulin （Ig） superfamily. It is expressed on the surface of activated T cells, regulatory T cells （Tregs）, natural killer （NK） cells, and some other immune cells. Structurally, it is similar to CD4 and has a higher binding affinity to MHC class II molecules than CD4. LAG-3 primarily functions as an immune checkpoint receptor, negatively regulating T-cell activation, proliferation, and cytokine production. In cancer and chronic infections, elevated LAG-3 expression is associated with T cell exhaustion, limiting anti-tumor and antiviral responses. Due to its significant role in immune regulation, LAG-3 has become a promising target for cancer immunotherapy and the treatment of autoimmune disorders.