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 gutassociated
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.