Lymphocyte Activation Gene-3 (LAG-3), also known as CD223. LAG-3 is a type I transmembrane protein and a member of the immunoglobulin superfamily, structurally related to CD4. It is expressed on activated and exhausted CD4+ and CD8+ T cells, regulatory T cells (Tregs), and some NK cells. LAG-3 acts as a co-inhibitory immune checkpoint receptor. Its binding to MHC-II molecules on antigen-presenting cells (APCs) or tumor cells delivers a potent negative signal that suppresses T-cell receptor (TCR) signaling, cytokine production, and T-cell proliferation. LAG-3 is frequently co-expressed with PD-1 on exhausted T cells within the tumor microenvironment, contributing to tumor immune escape.