Human Lymphocyte activation gene 3 protein( LAG3) is a member of immunoglobulin (Ig) superfamily. LAG3 contains 4 extracellular Ig-like domains. The LAG3 gene contains 8 exons. LAG3 is involved in lymphocyte activation and can bind to HLA class-II antigens. It is selectively expressed in activated T and NK cells. LAG3 has a negative regulatory function in T cells and acts as as a new marker of T cell induced B cell activation. As a soluble molecule, LAG3 activates antigen-presenting cells through MHC class II signaling. It can lead to increased antigen-specific T-cell responses in vivo. LAG-3 has higher affinity to MHC class II than CD4.