LOCALIZED LEUKOCYTE RESPONSE TO ICHTHYOPHTHIRIUS-MULTIFILIS ESTABLISHMENT IN IMMUNE CARP CYPRINUS-CARPIO L

O-group carp (Cyprinus carpio) which had been immunized against Ichthy ophthirius multifiliis by controlled infections were challenged by top ical application of theronts to the caudal fin. The parasites which es tablished were examined ultrastructurally, and host leucocyte response s were compared with those observed in primary infections. In the prim ary exposure group eosinophils and (to a lesser extent) basophils were the predominant cells infiltrating infection sites. In contrast, para site development in immunized fish initiated localized leucocytic infi ltrations which were dominated by eosinophilic granular cells (EGCs) a nd basophils. Greater localized phagocytosis was recorded in immunized fish by neutrophils, macrophages and resident epidermal filament cell s. In vitro studies indicated that pronephric leucocytes from immunize d fish displayed enhanced non-specific phagocytosis. In the skin, leuc ocytes were observed in close proximity to the trophozoite surface in both immunized and primary exposure fish, often undergoing lysis and r elease of cellular contents. However, there was no evidence of active cell adherence nor of any cell-mediated damage incurred to the parasit e in either case. These observations are discussed in relation to the possible role of leucocytes in mediating pathogenesis and immune respo nses.