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Ammonia (NH 3 ) is a hazardous gas that pollutes the environment and causes irritation. Its harmful effects on chickens, including its impact on their immune system, have previously been observed. However, the mechanism by which NH 3 exposure causes immune system disorders in chickens remains unclear. The bursa of Fabricius (BF) and thymus are the two main lymphoid organs responsible for the proliferation, differentiation, and selection of B- and T-lymphocytes, both critical for the innate immune response of the host. In this study, we investigated the mechanism of NH 3 -induced immune dysregulation in broiler chickens. Transmission electron microscopy (TEM) revealed swollen mitochondria and breakage of the large crista lining, membrane deformation, chromatin condensation, increased vacuolation, and blood vessel spasms in the NH 3 -exposed BF and thymus tissues. Immunofluorescent analysis showed clustering of CD4 + and CD8 + cells, indicating an active immune response to NH 3 exposure. Furthermore, NH 3 exposure enhanced the mRNA expressions of Toll-like receptor 7 (TLR-7), myeloid differentiation primary response 88 (MYD88), and nuclear factor-kappa B (NF-κB), along with their proteins, and led to activation of the TLR-7/MyD88/NF-κB signaling pathway and NLRP3 inflammasome in chicken thymus tissues. Both mRNA and protein levels of key inflammation-related genes and proteins were upregulated in the NH 3 -treated group, highlighting a robust inflammatory response due to NH 3 exposure. The specific findings of significant structural damage to key lymphoid organs and activation of inflammatory pathways in broiler chickens upon NH 3 exposure can provide guidance for future, targeted therapies to improve poultry health.