ABSTRACT
The spleen, thymus, and kidney are regarded as being the major immune organs in fishes, albeit with slightly various roles between species. The spleen is the main erythropoietic tissue in elasmobranchs, holocephalans, and a few teleosts. The teleosts lack lymph nodes and the spleen with the kidney form the two major filtering organs removing foreign agents. The spleen is composed of blood vessels, red and white pulps, and ellipsoids. The head kidney is a unique, important hemobiotic organ in fish. It contains more lymphocytes than the spleen and has been involved in antibody production. The kidney in teleost fish is the equivalent of the bone marrow in vertebrates and is the largest site of hematopoiesis until adulthood. The fish thymus is a paired lymphoid organ situated in the dorsal region of each branchial cavity. The parenchyma of the thymus is composed of several cell types, such as T lymphocytes, undifferentiated cells (lymphoblasts), melanomacrophages, nurse-like cells, immunoglobulin positive (Ig+) cells, mucous cells, rodlet cells, and neuroendocrine cells. The thymus acts as a site of production and differentiation of T lymphocytes (thymocytes) that are involved in cell-mediated immunity. Other studies described the appearance of focal epithelial nests, known as Hassal’s corpuscles.
The immune system of fish is physiologically similar to that of higher vertebrates, despite certain differences. In contrast to higher vertebrates, fish are free-living organisms from the early embryonic stages of life and depend on their innate immune system for survival. Nonspecific immunity is a fundamental defense mechanism in fish. In addition, it plays a key role in the acquired immune response and homeostasis through a system of receptor proteins.
The spleen, thymus, and kidney are regarded as being the major immune organs in fishes, albeit with slightly various roles between species. The spleen is the main erythropoietic tissue in elasmobranchs (sharks, rays), 84holocephalans (rabbitfish: Chimaera), and a few teleosts (Perca, Scorpaena). In most teleosts, in chondrosteans (sturgeon, paddlefish) and holosteans (gars, bowfin), erythrocytes are produced within the kidney. An extensive network exists for the trapping of blood-borne substances mainly in the kidney and spleen but in some species, other tissues such as the heart and liver are also involved. In the kidney and spleen, populations of lymphocytes and macrophages capable of mounting an immune response are situated close to sites of antigen trapping and often associated with accumulations of melanomacrophages (MMs). However, a primitive gut-associated lymphoid tissue represented by diffuse subepithelial lymphoid aggregates can be found. Species variation in the morphology of the immune system is to be expected, given the large number and diversity of species within the teleost fishes.
