Fish mucosal immunity: gill

The gills of fish are covered with a thin mucous membrane, which has a large external surface area that is in continuous contact with ambient water irrigating the organ during respiration. Large amounts of soluble and particulate substances, including various pathogens, may penetrate this mucosal barrier to cause local and/or systemic infections followed by mucosal and systemic immune responses. While substances that do not pose a threat to the fish may be tolerated by the gill immune system, pathogens have been found to induce innate and adaptive immune responses when entering the fish through the gills. Adaptive responses provide the basis for immune memory and consequently vaccination. Recently, investigations on the molecular level have added to our understanding of how immune responses of the gills are triggered. Such studies have heightened our understanding of host–pathogen interactions and are enhancing the development of immersion vaccines. This chapter describes teleost gill development, physiology, cellular composition, and immune responses to selected pathogens known to affect the gills.