LAG-3 (Lymphocyte Activation Gene-3) is a member of the immune checkpoint receptor family, primarily composed of four immunoglobulin-like domains. LAG-3 is mainly located on the surface of T cells, natural killer cells, and dendritic cells, participating in the regulation of immune responses and maintenance of immune tolerance. By binding to MHC II molecules, LAG-3 transmits inhibitory signals, suppressing T cell proliferation and cytokine production, thereby regulating T cell activity. In the tumor microenvironment, LAG-3 is upregulated, participating in immune evasion mechanisms. The specific expression and mutations of its gene are associated with the occurrence and development of various immune-related diseases and tumors.