96The 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. In most teleosts, erythrocytes are produced within the kidney. The teleosts lack lymph nodes, and the spleen with the kidney form the two major filtering organs removing foreign agents and effete blood cells from the vascular system. 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 spleen and has been shown to be actively involved in antibody production. The role of head-kidney as a major site of production of erythroid, lymphoid, and myeloid cells has been established. The kidney in teleost fish is the equivalent of the bone marrow in vertebrates and is the largest site of hematopoiesis until adulthood. Structurally, the anterior kidney is composed of a network of reticular fibers that provide support for lymph tissue. The fish thymus is a paired lymphoid organ situated in the dorsal region of each branchial cavity. Basically, the thymus can be considered as an aggregation of macrophages that promote the proliferation of T cells. The differentiation of the thymic structure is highly variable in teleosts, and in many species, it is not possible to observe a clear differentiation between the cortex and medulla that is found in higher vertebrates. Furthermore, myeloid cells and eosinophilic granular cells can be found in this organ. Other studies described the appearance of focal epithelial nests, known as Hassal’s corpuscles.