The morphology of the immune system in teleost fishes

An overview of the morphology of the immune system in teleost fishes is given drawing principally on studies in only a few species. The major lymphoid tissues in teleost fishes are the kidney, thymus, spleen and mucosa-associated lymphoid tissues including the skin and gills. The brief histological description of these tissues focuses on the microenvironmental compartments that are important for the defence systems of the fishes. In the thymus, a distinction between an outer cortex paked with thymocytes and a less densely populated inner medulla is possible in some species. The mucosal tissues including the gut, skin and gills provide a barrier to the entry of pathogens and contain leucocyte populations responsible for local immune responses. 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. 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.\C 1999 Academic Press