Feline 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 cats (Felis catus), CD40L is primarily expressed on activated CD4⁺ T lymphocytes and can also be found on platelets and other immune cells following activation. 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, immunoglobulin class switching, germinal center formation, and development of memory B cells. Through these interactions, feline CD40L plays a critical role in T cell–dependent antibody responses and coordination of adaptive immunity. In feline health, the CD40–CD40L signaling pathway is important for immune responses to viral infections such as feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) as well as bacterial and parasitic pathogens. Because CD40L is essential for T-cell–mediated B-cell help and immune regulation, feline CD40L is studied in veterinary immunology, infectious disease research, and vaccine development, helping to improve understanding of adaptive immune responses and immunomodulatory signaling in cats.