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The evolution of fish in an aqueous environment, with the large gills surface continuously exposed to pathogens, must have driven local mucosal immune responses to be extremely important for survival. In this study, we used a T/NK cells marker, ZAP70, and advanced imaging techniques to investigate the lymphoid architecture of the branchial cavity in adult zebrafish. With this approach, we identified a new lymphoid organ (NEMO), within the sub-pharyngeal region and in close association with gills. Besides T/NK cells, NEMO is also enriched in plasma/B cells and antigen-presenting cells that are embedded in a network of reticulated epithelial cells. While this organ is not associated with V(D)J recombination or hematopoiesis, it exhibits features of secondary lymphoid organs, such as the presence of activated T cells and lymphocyte proliferation. In response to viral and parasitic infections, NEMO displays …