The investigations presented in this thesis include studies on a) the immune responses of carp following direct immersion immunization and subsequent intraperitoneal (i.p.) challenge, b) the uptake and accumulation in carp of a direct immersion vaccine and c) phagocytic uptake by carp peritoneal exudate cells (PECs). To assess the cell-mediated immune response of carp, a micro chemotaxis technique was developed, measuring the production of chemotactic factor-like activity in supernatants from incubations of pronephric cells with antigen. In no case were serum antibody titres or a cell-mediated immune response detectable after immersions alone in antigen. It was found that an i.p. challenge of antigen in adjuvant, subsequent to the immersions, was needed to stimulate a measurable response, with effective priming immersions stimulating a secondary response to the i.p. challenge. It was found that the opsonization of both soluble and particulate immersion vaccines with immune carp serum significantly increased the immunological memory for both the humoral and the cell-mediated immune responses following immersion. Opsonization of the vaccines with normal serum, however, had no detectable effect. The cell-mediated immune responses following immersion were only measured in immunologically mature carp, but the humoral immune responses were measured in both immunologically mature and immature carp, which were 4 weeks old at the beginning of the experiments. Using the bacterial Aeromonas salmonicida antigen, all the responses measured post-immersion were found to be positive in both immunologically mature and immature carp. However, with the T-dependent antigen, human gamma globulin (HGG), the immune responses post-immersion were found to be positive only in the immunologically mature fish, with immersion of the immature carp in HGG-coated latex particles opsonized with immune serum producing a tolerizing effect on the humoral immune response. There was no detectable uptake of a non-opsonized A.salmonicida vaccine in normal carp when immersed in a bath of the vaccine. However, if the vaccine was opsonized with immune carp serum, uptake and accumulation of the vaccine was detectable, mostly accumulating in the internal lymphoid organs. Uptake of the non-opsonized vaccine was, however, also found when the recipient carp had been previously immunized against A.salmonicida, by immersion. The phagocytic uptake of particles by carp PECs was also found to be enhanced by opsonization of the particles with immune carp serum, this effect being partially recuced by decomplementation of the opsonizing serum. Opsonizat1on of particles with normal serum was found to have no effect on phagocytic uptake. Immersions in several different sizes of latex particles (from O.O5 µm to l5 µ.m) coated with HGG were found to stimulate greater humoral immunological memory than immersions in soluble HGG. This was not the case for memory for the cell-mediated response, where immersions in latex particle-bound HGG were no more stimulatory than immersions in soluble HGG. Carp PECs were found to be able to ingest 0.8 µm and 3.0 µm diameter particles but uptake of I5 µm diameter particles was not observed. The specificity of the humoral immune response after direct immersion immunization was found to be high with no cross-reactivity with any of the other antigens used. The cell-mediated immune response following direct immersion immunization was found to be slightly less specific; cross-reactivity between HGG and chicken gamma globulin was detected, although the other antigens used showed no cross-reactivity.

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