This study asks whether antigens encountered early in life by animals with free-living larvae can prime the immune system to yield memory responses on subsequent exposure and, if so, whether positive immunity or immunological tolerance is induced. In 4-week old carp, Cyprinus carpio, the thymus dependent antigen human gamma globulin (HGG) was tolerogenic both in soluble form and attached to latex particles, Formalin-killed Aeromonas salmonicida bacteria, in contrast, elicited primary antibody production and induced enhanced secondary responses. By the age of 9-10 weeks HGG was no longer tolerogenic even when injected into the thymus or administered orally. Despite their antibody tolerance fish primed with HGG at 4 weeks old resembled A. salmonicida-treated fish in their ability to elicit proliferative responses in the spleen and kidney after challenge, Direct immersion vaccination of 4-week old carp yielded enhanced antibody and proliferative responses on challenge with A. salmonicida. With particle-borne HGG it partially suppressed secondary antibody production but still yielded enhanced proliferation. Both carp and Xenopus laevis larvae were refractory to stimulation with soluble HGG by direct immersion. Mullet, Chelon labrosus, aged 6-7 months possessed well developed lymphoid organs but unlike adult fish required priming, orally or by injection, before they could respond to soluble antigens. In X. laevis, HGG induced positive memory rather than tolerance even in week-old larvae except when very high doses were employed such as induce tolerance in adults also. Injection of immune complexes into X. laevis toadlets resulted in accelerated antigen trapping and enhanced antibody production. Spleen cells which trap antigen can be separated on Percoll gradients and injected into non-immunized toadlets. This results in splenic antigen localization and accelerated antibody production. Foreign albumins were localized in the Xenopus spleen but yielded poor antibody production both in amphibians and fish.

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