The immune system
The immune system plays an important role in homeostasis by maintaining animal health in both innate and adaptive ways (Rice & Arkoosh 2002). Innate mechanisms are foundinagnathan and gnathostome fishes, and consist of immune factors that block invasion by potential pathogens. For example, the external layer of skin and scales is a physical barrier to infectious organisms. In addition, the sticky, viscous consistency of the mucus secreted by fish epithelial cells probably helps to trap microorganisms, and the mucus can contain antibodies and chemicals that destroy or inhibit bacteria (Bernstein et al. 1997). The volume of mucus secreted may increase in stressful situations, indicating a response on the fish’s part to shield itself from potentially harmful chemicals, microorganisms, or other agents. Other parts of the innate response include inducible phagocytic cells that can attack and destroy potential pathogens, cytotoxic cells that destroy cells infected by viruses, and the complement system of proteins that attack the membrane of invading cells (Rice & Arkoosh 2002).
The adaptive response, in contrast, involves the detection of an invader and the creation of specialized responsemechanisms to identify and destroy it. This response has not been seen in agnathans, but is present in the gnathostomes(Bernstein et al. 1997). The organs primarily responsible for this response are the kidney, thymus, spleen, and gut.
The adaptive response includes both cellular and humoral components (Rice & Arkoosh 2002). The cellularcomponent of the adaptive response includes cytotoxic Tells that can destroy cells that have become infected by viruses or that show signs of becoming cancerous. The humoral response involves the detection of specific invading compounds (antigens) and the production of antibodies designed to bind to them. These antibodies tag the antigenic particles for destruction by other components of the immune system, such as macrophages that engulf and digest the tagged antigens, or complement proteins that destroy tagged cells by puncturing their membranes. Antibody structure of the Chondrichthyes is somewhat similar to that of the higher bony fishes and mammals. The structures of the genes responsible for antibodies are quite different, however, with those of the bony fishes somewhat intermediate between those of the Chondrichthyes and those of mammals (Bernstein et al. 1997).
In mounting an antibody response, the immune system also produces memory cells that remain in the blood stream for extended periods (Rice & Arkoosh 2002). Memory cells help the animal’s immune system react quickly if it encounters the same antigen in the future. Consequently, subsequent exposures to an antigen are dealt with quickly and the antigens are destroyed much more quickly than waste case during the initial exposure to that same antigen. Vaccinations, which have become important in fish culture, take advantage of memory cell development. By exposing fish to a less virulent form of a pathogen, the fish’s immune system can defeat this initial infection and will retain memory cells to help it respond quickly and more effectively to subsequent exposures to a potentially more virulent form of the pathogen.