Immune System of Fish

The characteristics of the fish defense system, its relationship with environmental conditions and other factors are presented. In contrast to higher vertebrates the antibody response of fish is weaker and arises with a certain delay. The reaction can vary depending on the place in the body of a fish. There is a great diversity of lymphoid and lymphomyeloid organs in fish. In addition to the thymus, kidney and spleen, they include the Leydig organ, a tissue associated with the esophagus, the epigonal organ, linked to the gonads, the pericardial tissue, the orbital and the preorbital tissues, that are the tissues located in the region of the fish eyes. The mucosa-associated lymphoid tissue of fish is characterized by the lack of organized structures, dominance of specialized mucosa-IgT immunoglobulins and IgT + B cells, a variety of symbiotic bacteria that are overlaid by mucosal Igs of fish. Attention is paid to the interbranchial lymphoid tissue which was found only in salmonids fish and cutaneous immune system observed only in fish and amphibians. The role of various organs (pseudobranch, nasopharynx) and cells involved in immune reaction in fish is discussed.