Atlantic Salmon (Salmo salar) Interleukin-6 (IL-6) is a pro-inflammatory cytokine that plays a central role in regulating innate and adaptive immune responses in teleost fish. Although structurally divergent from mammalian IL-6, salmon IL-6 shares functional similarities, including activation of JAK/STAT signaling pathways through gp130-like receptor components and induction of acute-phase responses, leukocyte activation, and inflammatory gene expression. In Atlantic salmon, IL-6 expression is rapidly upregulated in response to bacterial infections such as Aeromonas salmonicida and Vibrio species, viral pathogens including infectious salmon anemia virus (ISAV) and piscine orthoreovirus (PRV), and parasitic challenges such as sea lice (Lepeophtheirus salmonis). Elevated IL-6 levels are associated with systemic inflammation, modulation of macrophage and lymphocyte activity, and coordination of mucosal immune responses in gill, skin, and gut tissues—key barriers in aquatic environments. As an important biomarker of immune activation, salmon IL-6 is widely used in aquaculture research to evaluate vaccine efficacy, disease resistance, stress responses, and the impact of environmental conditions on fish health. Characterization of Atlantic salmon IL-6 supports improved understanding of teleost immunology, host-pathogen interactions in aquatic systems, and development of immunomodulatory strategies to enhance fish welfare, productivity, and sustainability in commercial aquaculture.