Abstract
<title>Abstract</title> <p>β-glucan stimulates systemic immunity, but its effects on mucosal immune regulation remain unclear, including whether it triggers non-specific activation that could disrupt beneficial microbiota. Therefore, this study aimed to investigate how β-glucan modulates mucosal immunity in fish, focusing on local molecular signaling and gene expression profiles in gut-associated lymphoid tissue (GALT) and gill-associated lymphoid tissue (GIALT), as well as its systemic effects. The experiment followed a fully randomized 2x2 factorial design (control diet vs. 0.1% β-glucan; sampling at 15 and 30 days). The expression of cytokine genes in the intestine and gills, serum immunological markers, were evaluated. β-glucan supplementation increased IL-8 and IL-10 expression in the intestine after 30 days, while reducing IL-6, IFN-γ, and HSP70 expression in the gills. These findings suggest that β-glucan is associated with a lower inflammatory/stress-related transcriptional profile in intestinal and gill mucosae, potentially contributing to the maintenance of tissue homeostasis and mucosal integrity. Additionally, in the bloodstream, β-glucan supplementation elevated lysozyme concentration and hemolytic activity of the alternative complement pathway, indicating an enhancement of systemic innate immune responses. These results demonstrate that β-glucan modulates both local and systemic immune functions in fish, enhancing humoral mechanisms against pathogens while supporting a more balanced mucosal immune profile. Future studies integrating histology, oxidative markers, and microbiome profiling should help clarify the underlying mechanisms.</p>