Chicken 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 chickens (Gallus gallus), CD40L is primarily expressed on activated CD4⁺ T lymphocytes, and can also be detected on other activated immune cells following antigen stimulation. CD40L interacts with its receptor CD40, which is expressed on B cells, macrophages, dendritic cells, and other antigen-presenting cells, triggering intracellular signaling pathways such as NF-κB and MAPK that promote B-cell activation, proliferation, antibody production, and germinal center–like responses in avian lymphoid tissues. Through these mechanisms, chicken CD40L plays a crucial role in T cell–dependent humoral immunity and regulation of adaptive immune responses. In poultry health, the CD40–CD40L signaling pathway contributes to immune defense against viral, bacterial, and parasitic pathogens, including avian influenza virus, Newcastle disease virus, infectious bursal disease virus, and Eimeria species. Because CD40L is important for antibody responses and immune regulation in birds, chicken CD40L is widely studied in avian immunology, vaccine development, and host–pathogen interaction research, helping improve strategies for disease control and immune protection in poultry.