Histological and immunological parameters were monitored in carp during the primary immune response against Aeromonas hydrophila. Localization of antigen in the spleen initially ocurred in the ellipsoids, whereas in the head and trunk kidney it was diffusely distributed. When antibody titers increased (from day 9 onwards), antigen accumulated in melano-macrophage clusters in all three organs, and it was mainly bound to cell membranes. Tissue-bound immunoglobulin was present on several structures in the lymphoid organs. As a consequence of immunization, the number of cells with cytoplasmic immunoglobulin and the amount of immunoglobulin associated with the reticular sheath of ellipsoids and membranes of melano-macrophages increased. Pyroninophilia was limited, and occurred in the spleen associated with ellipsoids; in addition, clusters of cells appeared, displaying a small rim of pyroninophilic cytoplasm. In the kidney the reaction was not conspicuous. The number of antibody forming cells in the head kidney peaked at day 9 and the serum antibody response followed with a peak at day 15. When the antigen was emulsified in adjuvant, the antigen processing and the antibody responses were affected. Antigen processing was retarded and a second burst of antibody production occurred from day 15 onwards. The relationship of melano-macrophage centers with immunological processes is considered. It is suggested that in fish these centers might be primitive analogues of the mammalian germinal centers.
Dr. Ir. Lamer's current address is, Molecular Oncology Sec-