Rat CD40 Ligand (CD40L), also known as CD154 and encoded by the Cd40lg gene, is a type II transmembrane protein belonging to the tumor necrosis factor (TNF) superfamily, which includes related ligands such as TNF-α, CD27 ligand (CD70), OX40 ligand (CD252), and 4-1BB ligand (CD137L) that regulate immune activation and intercellular signaling. In rats (Rattus norvegicus), CD40L is primarily expressed on activated CD4⁺ T lymphocytes, and can also be detected on platelets and other activated immune cells following antigen stimulation. CD40L binds to its receptor CD40, which is expressed on B cells, dendritic cells, macrophages, and other antigen-presenting cells, triggering intracellular signaling pathways such as NF-κB and MAPK that promote B-cell activation, proliferation, immunoglobulin class switching, germinal center formation, and the development of memory B cells. Through these mechanisms, rat CD40L plays a key role in T cell–dependent antibody responses and regulation of adaptive immunity. Because rats are widely used as experimental models for immunology, autoimmune disease, transplantation, and inflammatory disorders, rat CD40L has been studied in models of immune activation, chronic inflammation, and immune-mediated diseases, helping to clarify mechanisms of costimulatory signaling and adaptive immune regulation relevant to both basic immunology and translational biomedical research.