Lymphocyte activation gene 3 protein: Inhibitory receptor on antigen activated T-cells (PubMed:7805750, PubMed:8647185, PubMed:20421648). Delivers inhibitory signals upon binding to ligands, such as FGL1 (By similarity). FGL1 constitutes a major ligand of LAG3 and is responsible for LAG3 T-cell inhibitory function (By similarity). Following TCR engagement, LAG3 associates with CD3-TCR in the immunological synapse and directly inhibits T-cell activation (By similarity). May inhibit antigen-specific T-cell activation in synergy with PDCD1/PD-1, possibly by acting as a coreceptor for PDCD1/PD-1 (By similarity). Negatively regulates the proliferation, activation, effector function and homeostasis of both CD8(+) and CD4(+) T-cells (PubMed:7805750, PubMed:8647185, PubMed:20421648). Also mediates immune tolerance: constitutively expressed on a subset of regulatory T-cells (Tregs) and contributes to their suppressive function (By similarity). Also acts as a negative regulator of plasmacytoid dendritic cell (pDCs) activation (By similarity). Binds MHC class II (MHC-II); the precise role of MHC-II-binding is however unclear (PubMed:8647185). 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 (PubMed:1692078, PubMed:7589152, PubMed:8647185, PubMed:9159144). Interacts with FGL1 (via the Fibrinogen C-terminal domain) (PubMed:30580966). Produced following cleavage of the main chain. Primarily expressed in activated T-cells and a subset of natural killer (NK) cells. Expression is induced by interleukin-2 (IL2), interleukin-7 (IL7) and interleukin-12 (IL12A and IL12B) on activated T-cells. 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.