The innate and adaptive immune system of fish

Abstract This chapter describes what is known about the main components and responses of the innate and adaptive immune system of fish. The chapter first reviews the organs, cells and molecules of the immune system known in a few economically important or model fish species. Molecular evidence suggests a similar immune system exists throughout the jawed vertebrates yet marked differences are also apparent. The innate parameters are at the forefront of fish immune defence and are a crucial factor in disease resistance. The adaptive response of fish is commonly delayed but is essential for long lasting immunity and a key factor in successful vaccination. 

Key words fish immune organs, pattern recognition receptors (PRR), innate immune responses, adaptive immune responses, immune regulation. 

However, there is great variation in disease susceptibility and immune defence between different fish species, a reflection of the extended time the present day teleosts have been separated during the evolution of this fish group. Thus the immune response described in one species may not be the 

Immune responses are mediated by a variety of cells and secreted soluble mediators. Leucocytes are central to all immune responses, and include lymphocytes (T cells, B cells, large granular lymphocytes), phagocytes (mononuclear phagocytes, neutrophils and eosinophils) and auxiliary cells (basophils, mast cells, platelets). Other cells in tissues also participate in the immune responses by signalling to the leucocytes and responding to the soluble mediators (cytokines) released by leucocytes such as T cells and macrophages. 

The cells involved in the immune responses are organized into tissues and organs in order to perform their functions most effectively. These structures are collectively referred to as the lymphoid system, and are arranged into either discretely encapsulated organs or accumulations of diffuse lymphoid tissue. The major lymphoid organs and tissues are classified as either primary (central) or secondary (peripheral). Lymphocytes are produced in the primary lymphoid organs and function within the secondary lymphoid organs and tissues. 

---

The innate and adaptive immune system of fish 39 T cell immune response... 

---