Lymphocyte activation gene 3 protein: Inhibitory receptor on antigen activated T-cells. Delivers inhibitory signals upon binding to ligands, such as FGL1. FGL1 constitutes a major ligand of LAG3 and is responsible for LAG3 T-cell inhibitory function. Following TCR engagement, LAG3 associates with CD3-TCR in the immunological synapse and directly inhibits T-cell activation. May inhibit antigen-specific T-cell activation in synergy with PDCD1/PD-1, possibly by acting as a coreceptor for PDCD1/PD-1. Negatively regulates the proliferation, activation, effector function and homeostasis of both CD8(+) and CD4(+) T-cells. Also mediates immune tolerance: constitutively expressed on a subset of regulatory T-cells (Tregs) and contributes to their suppressive function. Also acts as a negative regulator of plasmacytoid dendritic cell (pDCs) activation. Binds MHC class II (MHC-II); the precise role of MHC-II-binding is however unclear. May function as a ligand for MHC class II (MHC-II) on antigen-presenting cells (APC), promoting APC activation/maturation and driving Th1 immune response. Interacts with MHC class II (MHC-II); selectively recognizes stable complexes of peptide and MHC-II. Interacts with FGL1 (via the Fibrinogen C-terminal domain). Produced following cleavage of the main chain. The KIEELE motif is required for interaction with downstream signaling molecules. Proteolytically cleaved by ADAM10 and ADAM17 within the connecting peptide region, leading to release of Secreted lymphocyte activation gene 3 protein (sLAG-3). ADAM10 mediates constitutive cleavage, but cleavage increases following T-cell activation, whereas shedding by ADAM17 is induced by TCR signaling in a PRKCQ-dependent manner. Belongs to the LAG3 family.