Ovine 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 intercellular signaling. In sheep (Ovis aries), 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, initiating intracellular signaling pathways such as NF-κB and MAPK that promote B-cell activation, proliferation, immunoglobulin class switching, germinal center formation, and development of memory B cells. Through these mechanisms, ovine CD40L plays a critical role in T cell–dependent antibody responses and regulation of adaptive immunity. In sheep, the CD40–CD40L signaling pathway contributes to immune responses against viral, bacterial, and parasitic pathogens, including those associated with respiratory disease, gastrointestinal infections, and parasitic infestations common in small ruminant production systems. Because effective humoral immunity is essential for disease resistance and vaccine-induced protection, ovine CD40L is studied in veterinary immunology, vaccine development, and host–pathogen interaction research, helping to improve strategies for immune protection and disease control in sheep.