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.; [Secreted lymphocyte activation gene 3 protein]: 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.