Antigen localization in the lymphoid organs of carp (Cyprinus carpio).

A brief morphological description is given of the spleen, head kidney and trunk kidney, the three peripheral lymphoid organs of carp. An antigen-localization study was carried out using Aeromonas hydrophila (a cellular bacterial antigen) and an indirect immunofluorescence test. Examination of the lymphoid organs at various times after injection (up to 12 months) showed that antigen was at first present in splenic ellipsoids and in solitary phagocytic cells in the spleen, head and trunk kidney. Later on, the antigen was gradually concentrated in or near melano-macrophage centres in all the organs studied. By this time, it had disappeared from the splenic ellipsoids, and the number of solitary A. hydrophila-immunoreactive cells had also decreased. At a later stage, antigen was located extracellularly at the membrane of cells in and around the melano-macrophage centres, and it remained so for a full year. No antigen was detected in the lymphoid organs following a bath immunization.