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Defenses against disease

Fish have both non-specific and immune defenses against disease. Non-specific defenses include the skin and scales, as well as the mucus layer secreted by the epidermis that traps and inhibits the growth of microorganisms. If pathogens breach these defenses, the innate immune system can mount an inflammatory response that increases blood flow to the infected region and delivers white blood cells that attempt to destroy pathogens, non-specifically. Specific defenses respond to particular antigens, such as proteins on the surfaces of pathogenic bacteria, recognised by the adaptive immune system. Immune systems evolved in deuterostomes as shown in the cladogram.

Immune organs vary by type of fish. The jawless fish have lymphoid tissue within the anterior kidney, and granulocytes in the gut. They have their own type of adaptive immune system; it makes use of variable lymphocyte receptors (VLR) to generate immunity to a wide range of antigens, The result is much like that of jawed fishes and tetrapods, but it may have evolved separately. All jawed fishes have an adaptive immune system with B and T lymphocytes bearing immunoglobulins and T cell receptors respectively. This makes use of Variable–Diversity–Joining rearrangement (V(D)J) to create immunity to a wide range of antigens. This system evolved once and is basal to the jawed vertebrate clade. Cartilaginous fish have three specialized organs that contain immune system cells: the epigonal organs around the gonads, Leydig's organ within the esophagus, and a spiral valve in their intestine, while their thymus and spleen have similar functions to those of the same organs in the immune systems of tetrapods. Teleosts have lymphocytes in the thymus, and other immune cells in the spleen and other organs.