Antibody responses to soluble and cellular antigens have been demonstrated in juvenile and adult plaice, Pleuronectes platessa L. A relatively strong precipitating antibody was produced to calf serum, but not to bovine serum albumin. The onset, and magnitude of antibody production was temperature dependant and physico-chemical characterisation of plaice precipitins indicated they were of the IgM class. Plaice also produced haemagglutinins, following injection of rabbit erythrocytes, with a suggestion of a heightened response upon secondary stimulation with the antigen. An evaluation was made of the humoral immune.response of the fish to three tissue parasites; Rhipidocotyle johnstonei and Cryptocotyle lingua, the metacercariae of which develop in the musculature and. connective tissue of plaice; and Trypanosoma platessae, a haemoflagellate. Elevated levels of β-globulin were detected in the sera of T. platessae infected plaice, and it is suggested that this may be associated with antibody secretion to the parasite. Further studies were made on the biology of T. platessae, including efforts to maintain the parasite, by passage, in the laboratory. Natural and experimental infections with the metacercariae of C. lingua and R. johnstonei induced a temperature dependant, precipitin response in plaice. Application of the indirect fluorescent antibody technique suggested that the functional antigens of both parasites included somatic elements, however the antigens of R. johnstonei were also clearly associated with secretory tissues. A 'natural' antibody, present in the sera of the majority of test plaice, was found to precipitate in agar gel with an antigenic extract of the nematode, Proleptus obtusus. This was considered of particular interest, as P. obtusus is a parasite specific to the dogfish and is unknown in plaice. The precipitin, first suspected of being non-specific C-reactive protein, was later characterised as 19S IgM. The nature of serological and immunological changes in fish subjected to immunisation and. parasitic infection was reviewed and discussed in relation to the findings of the present project.

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