An overview of the structural and functional aspects of immune cells in teleosts.

The immune system of fish consists of two main components, innate and adaptive immunities. Innate immunity is non-specific and acts as the primary line of protection against pathogen invasion, while adaptive immunity is more specific to a certain pathogen/following adaptation. The adaptive immune system consists of the humoral and cellular components. Cytotoxic T-lymphocyte cells are the major component of the cellular immunity that frequently kills viral-, bacterial- or parasitic-infected cells. According to the anatomical location, the mucosal-associated lymphoid tissue (MALT) in teleost fish subdivides into gut-associated lymphoid tissue (GALT), gill-associated lymphoid tissue (GIALT), and skin-associated lymphoid tissue (SALT). The MALTs contain various leukocytes; including, but not limited to, lymphocytes (T and B cells), plasma cells, macrophages, and granulocytes. Macrophages are multifunctional cells that are mainly involved in the immune response, including; phagocytosis and degradation of foreign antigens, tissue remodeling, and production of cytokines, chemokines and growth factors. An interesting feature of teleost macrophages is their ability to form melanomacrophage centers (MMC) in the hemopoietic tissues. Dendritic cells, rodlet cells, mast cells, eosinophilic granular cells (ECGs), telocytes, osteoclasts, club cells, as well as, barrier cells have been recorded in many fish species and have many immunological roles. This paper aims to summarize the current knowledge of the immune cells present in fish tissues serving as anatomical and physiological barriers against external hazards. Increased knowledge of fish immune systems will facilitate the development of novel vaccination strategies in fish.