CD223 also known as LAG-3 (Lymphocyte-activation gene 3) is a 70 kDa type I transmembrane protein with one modified V-set Ig superfamily domain followed by 3 C2-set Ig domains. It has homology with CD4. CD223 is expressed on activated T and NK cells, expressed at higher levels on CD8 than CD4 cells, on activated B cells and plasmacytoid dendritic cells. LAG-3 also has been reported to be specifically expressed on regulatory T cells of the Tr1 type.



CD223 binds to MHC class II and regulates the expansion of antigen-specific immune responses. LAG-3 is an inhibitory checkpoint which plays an important role in modulating T cell expansion and function. In addition, the interaction between LAG-3 and its major ligand, Class II MHC, is thought to play a role in modulating dendritic cell function.