The fish immune system consists of innate and adaptive immunity, which together provide defence against waterborne pathogens. Innate immunity is more pronounced in fish than in mammals, as adaptive responses develop slowly and are less effective. The first line of defence consists of physical barriers and mucosal surfaces (skin, gills, gut), which block pathogen entry. Innate immunity includes cellular components (eosinophil granular cells, neutrophils, macrophages, dendritic cells, nonspecific cytotoxic cells) and humoral factors (lysozyme, transferrin, lectins, C-reactive protein, complement). These ensure rapid recognition and elimination of microbes through phagocytosis, inflammation, and lysis. Adaptive immunity relies on T and B lymphocytes, as well as antibody production. The main isotype is tetrameric IgM (abundant in serum); others are IgD and IgT (role in mucosal activity). Its essential features are specificity, mediated by V(D)J recombination and somatic hypermutation, and immunological memory. Compared to mammals, fish antibodies display lower variability and affinity and lack certain molecules (such as DM), yet still perform neutralisation, complement activation, and opsonisation. A distinctive feature of fish is the variability of immune responses, strongly influenced by external factors (temperature, salinity). Despite differences from mammals, the fish immune system preserves evolutionary features that ensure coordination between innate and adaptive immunity and provide effective, long-lasting protection.
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