Regulation of The Immune System of Fish to Viral Infections

In fish, lymphoid tissues and organs are divided into two groups as primary and secondary. The primary lymphoid tissues and organs consist of thymus, kidney, spleen while secondary ones consist of liver, skin and intestines. Although bone marrow and lymph nodes do not exist in fish, most of immunological organs and cells along with immune system have the same structure and function as the mammals. In fish, there are also two different responses of immune system as natural and acquired. Natural immunity is stimulated expeditiously following the infection, it is characterized with absence of immunologic memory. It is not antigen specific stimulus but is modulated by the molecules encoded with genes. Acquired immunity has an excellent specifity and memory; the key humoral parameter is the antibodies expressed by the receptors of B lymphocytes. When the pathogens pass over the natural defence mechanism, the acquired immune system begins activation and specific lymphocytes proliferate and differentiate into long term memory cells and plasma cells. The organism would exhibit a rapid and effective immune response upon subsequent exposures to these pathogens. In the defence of fish against viral pathogens, both natural and acquired immune system work interactively. In this review, the information about the lymphoid organs and tissues of fish, their immune system cells and mediators and functioning of immune system in case of viral infections are reported.