Toll-like receptor-4 in the fish immune system.

Toll-like receptors (TLRs) represent a critical class of protein molecules integral to nonspecific immunity (innate immunity), and serve as key mediators bridging between nonspecific and specific immune responses. As an important member of the TLR multigene family, toll-like receptor 4 (TLR4) holds particular significance due to its broad expression across various animal tissues and its pivotal roles in pathogen recognition, signal transduction, and the modulation of inflammatory responses. In aquaculture, Tlr4 has emerged as a promising target for the development of vaccines, immune enhancers, and adjuvants aimed at bolstering the health and disease resistance of aquatic animals, thereby reducing the reliance on antibiotics. To date, studies on Tlr4 primarily focused on its structural and functional properties, expression patterns, immune responses, and evolutionary dynamics. This review synthesizes current knowledge on the structural features, functions, regulatory mechanisms, and potential applications of Tlr4 in the immune system of fish, providing a foundational framework for advancing studies on fish immunity.