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Melanomacrophages (MMs) are phagocytizing cells with high amounts of pigments including melanin which can be found in a number of cold-blooded species. In Osteichthyes, these cells cluster to form so-called melanomacrophage centres (MMCs), which are predominantly present in the stroma of hematopoietic and lymphoid tissues, that is, in the kidney and spleen. The functionality of MMs and MMCs results from their involvement and role in the defence reactions, related to both the innate and the adaptive immune mechanisms, and in processes unrelated to defence functions as well. There is evidence that MMCs are structurally and functionally similar to mammals' germinal centres (GCs). It appears that mature IgM+ B cells in Osteichthyes can be the equivalent of mIgM+ centrocytes in mammals, whereas MMs can be, in terms of the function, the equivalent of follicular dendritic cells (FDCs), and MMCs can be, in terms of clustered specific cells, the equivalent of GCs. This paper presents selected facts about the structural and functional similarity between GCs and MMCs and about the involvement and role of MMCs and MMs in the immune response. The facts help get a proper picture of the location of MMs and MMCs within the structure of the fish immune system, also in the context of their evolutionary relationship with GCs and of the possibility of pointing out the evolutionary closeness between MMCs in Osteichthyes and GCs in mammals., Competing Interests: The authors declare no conflict of interest.

The fish immune system is extremely complex and has considerable adaptive potential. In Osteichthyes, the system is formed by lymphopoietic organs which are important for the differentiation and maturation of the immune system cells. These organs include the anterior kidney (phronephros), the thymus, the spleen, the posterior kidney (mesonephros), and mucosa-associated lymphoid tissues (MALT). Apart from the lymphocytic organs and the MALT system, the immune system components include defensive cells and their products. Those identified in fish include, inter alia, monocytes/macrophages, melanomacrophages, neutrophilic granulocytes, thrombocytes, B cells, plasma cells, and T cells. The roles of the individual components of the organisation of the immune system, the organs, and lymphoid tissue as well as the constituents conditioning the innate and adaptive immunity mechanisms are considered equally important, especially in the context of functional interdependence. The progress in the exploration of the processes of specific humoral immunity in Osteichthyes and the possibilities of their practical application is increasingly promising in view of the expected need for protection of fish against diseases. The paper discusses selected issues concerning recent knowledge about haematopoiesis of B cells, plasmablasts, plasma cells, and immunoglobulins (IgM, IgD, IgT/IgZ)., Competing Interests: The authors declare no conflict of interest.