The vertebrate immune system is comprised of numerous distinct and interdependent components. Every component has its own inherent protective value, and the final combination of them is likely to be related to an animal’s immunological history and evolutionary development. Vertebrate immune system consists of both systemic and mucosal immune compartments, but it is the mucosal immune system which protects the body from the first encounter of pathogens. According to anatomical location, the mucosa-associated lymphoid tissue, in teleost fish is subdivided into gut-, skin-, and gill-associated lymphoid tissue and most available studies focus on gut. The purpose of this paper is to summarise the current knowledge of the immunological defences present in skin mucosa as a very important part of the fish immune system, serving as an anatomical and physiological barrier against external hazards. Interest in defence mechanism of fish arises from a need to develop health management tools to support a growing finfish aquaculture industry, while at the same time addressing questions concerning origins and evolution of immunity in vertebrates. Increased knowledge of fish mucosal immune system will facilitate the development of novel vaccination strategies in fish.

The aquaculture industry has dramatically developed during the last two decades. However, this development has, in some cases, resulted in environmental degradation, emergence of diseases, and low productivity. The need for improving disease resistance, growth performance, feed efficiency, and safe aquatic production for human consumption has stimulated development and applications of probiotics in aquaculture. Probiotics used in aquaculture include genera of Bacillus, Lactobacillus, Enterococcus, and Carnobacterium, and yeast. However, most of these probiotics are derived from terrestrial sources and not from the environment in which the aquatic animals live or the host animal. The use of “host-associated probiotics” has gained attention, as they offer an alternative strategy within aquaculture, which per se is dependent on the use of terrestrial microorganisms. The benefits of host-associated probiotics include improved growth performance, feed value, enzymatic contribution to digestion, inhibit adherence, and colonization of pathogenic microorganisms in the gastrointestinal tract, increase hematological parameters, and immune response. The present review addressed insight into the application of host-associated probiotics within aquaculture, with special focus on their immunomodulatory and growth enhancing effects. Furthermore, the current review address research gaps and issues that merit further investigations.