Swine 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 cell–cell signaling. In pigs (Sus scrofa), 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, swine CD40L plays a key role in T cell–dependent antibody responses and regulation of adaptive immunity. In swine health, the CD40–CD40L signaling pathway contributes to immune responses against viral and bacterial pathogens, including porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus, and other infectious agents affecting pigs. Because pigs share many physiological and immunological similarities with humans, swine CD40L is also studied in comparative immunology, vaccine development, and translational animal models, helping to improve understanding of costimulatory signaling and immune regulation in both veterinary and human medicine.