SPECIAL STORY FEED & ADDITIVE MAGAZINE November 2023 71 aquaculture, because fish lack the ability to migrate to optimal environmental conditions. This increases their susceptibility to disease expression. The immune system can be subdivided into three lines of defense against the various forms of pathogens. FIRST LINE OF DEFENSE: BLOCKING PATHOGENS AND FOREIGN MATERIALS FROM ENTERING THE BODY A protective mucus layer covers the entire surface area of physical barriers, such as the skin, gills and gut. Diverse microbiota optimize health through the competitive exclusion of bacteria, which reduces the risk of pathogens taking hold of the microbiome. SECOND LINE OF DEFENSE: THE INNATE IMMUNE SYSTEM The activation of the innate immune system takes place when pathogens successfully pass through external barriers and attempt to infect the organism. The innate immune system attacks invading pathogens using white blood cells (leukocytes), which can differentiate between “self” and “non-self” cells. It targets cells that lack the recognizable marker molecules of the body. Within this system, immune activity is further regulated by inflammation and antimicrobial proteins. THIRD LINE OF DEFENSE: THE ADAPTIVE IMMUNE SYSTEM The adaptive immune system differentiates itself from the innate immune system by stimulating a pathogen-specific immune response. It singles out and eradicates a single pathogen, earning the title “specific immune system.” This response is also known as the secondary immune response and forms the foundation for vaccinating many animal species. Figure 1. The first line of defense prevents pathogens from entering the body by mucus layers and cell barriers (from left to right, the protective tissue and mucus layer of the skin, gills and gut) Image 1. Overview of the immune system
RkJQdWJsaXNoZXIy MTUxNjkxNQ==