Bovine CD40 Ligand (CD40L), also known as CD154 and encoded by the CD40LG gene, is a type II transmembrane protein that belongs 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 cattle (Bos taurus), CD40L is primarily expressed on activated CD4⁺ T lymphocytes, and can also be detected on platelets and other activated immune cells. CD40L binds to its receptor CD40, which is expressed on B cells, dendritic cells, macrophages, and other antigen-presenting cells, initiating intracellular signaling pathways such as NF-κB and MAPK that promote B-cell activation, immunoglobulin class switching, germinal center formation, and the development of memory B cells. Through these mechanisms, bovine CD40L plays a central role in T cell–dependent antibody responses and regulation of adaptive immunity. In cattle, the CD40–CD40L signaling pathway contributes to immune responses against viral, bacterial, and parasitic pathogens, including those involved in bovine respiratory disease, mastitis, and other infectious conditions. Because CD40L is critical for T-cell–mediated B-cell help and effective vaccine-induced immunity, bovine CD40L is studied in veterinary immunology, vaccine development, and host–pathogen interaction research, helping to improve strategies for disease control and immune protection in cattle.