LAG-3 (Lymphocyte-Activation Gene 3), a type I transmembrane protein, comprises an extracellular region with four immunoglobulin (Ig)-like domains (D1-D4), a transmembrane segment, and a cytoplasmic tail. The D1 domain contains a unique 30-amino acid "extra loop" that mediates interactions with major histocompatibility complex class II (MHC II) molecules through distinct binding mechanisms compared to its structural homolog CD4, despite sharing approximately 20% sequence homology in extracellular regions. This loop, conserved across humans and mice, contributes to LAG-3's stronger MHC II affinity while exhibiting species-specific conformational plasticity. The D2-D4 domains harbor multiple N-glycosylation sites that facilitate interactions with alternative ligands like Galectin-3 and LSECtin through carbohydrate recognition. Notably, the "connecting peptide" between D4 and the transmembrane region serves as a cleavage site for ADAM10/17 metalloproteases in mice, potentially regulating surface LAG-3 expression, though human relevance remains unclear. Functionally, LAG-3 acts as a negative immune checkpoint regulator, primarily expressed on activated T cells (CD4+/CD8+), NK cells, and dendritic cells. Its cytoplasmic region features three conserved motifs: a KIEELE sequence with unknown function, a serine phosphorylation site (S454) for kinase signaling, and EP repeats critical for intracellular signal transduction. Through multiple ligands including MHC II, FGL1, and Galectin-3, LAG-3 suppresses T cell activation and promotes regulatory T cell (Treg) immunosuppressive activity.