THE RESPONSES OF MULLET, CHELON LABROSUS, AND CARP, CYPRINUS CARPIO, TO ANTIGEN ADMINISTERED BY INJECTION OR DlRECT IMMERSION

The nature of humoral non-reactivity in the grey mullet, Chelon labrosus, and carp, Cyprinus carpio L., to thymus dependent protein antigens was investigated with reference to age, route of administration and by the use of potentially immunosuppressive drugs. The effects of primary and challenge administrations of antigen upon memory induction in both species were also studied. Juvenile grey mullet were unable to respond humorally to a single injection of the T-dependent antigen human gamma globulin (HGG); they could however produce antibodies after challenge. Formalin killed Aeromonas salmonicida bacteria elicited a primary antibody response, but there was no evidence of enhancement on challenge. Immunization by direct immersion did not elicit antibodies to either antigen, even after challenge. There was however an increase in the numbers of splenic melano-macrophage centres after immunization regardless of antigen type or route. There was evidence that both soluble and latex-bound HGG were taken up across the gills of the mullet following direct immersion; radio-isotope labelled A.salmonicida was seen within the gills and in the lumen of the anterior gut. There was no evidence of antigen uptake in the anterior gut. The administration of a corticosteroid did not effect either the humoral or splenic response to priming or challenge with HGG in the juvenile mullet. The primary humoral response to A.salmonicida was reduced by the administration of the corticosteroid, the splenic response was unaffected. There was no evidence that the corticosteroid was cytotoxic in vitro. Carp, 9 to 10 weeks old, were unable to respond humorally to ovalbumin (OVA), their proliferative responses were however comparable to fish injected with HGG, which could respond by anti-HGG antibody production. At 3 months old the carp were able to respond humorally to two direct immersions of either HGG or A.salmonicida. There was no evidence of enhancement of the humoral response in older carp following priming and challenge with A.salmonicida by either direct immersion or injection; there was an increase in the splenic response following priming and challenge by direct immersion