LAG3, also known as CD223 and Lymphocyte activation gene 3, belongs to immunoglobulin (Ig) superfamily. LAG3 contains 4 extracellular Ig-like domains. The LAG3 gene contains 8 exons. It is selectively expressed in activated T and NK cells. LAG3 has a negative regulatory function in T cells. It also 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 signalling, leading to increased antigen-specific T-cell responses in vivo.