The humoral immune response of roach (Rutilus rutilus)
to cercariae of the digenean trematode, Rhipidocotyle fennica,
was
studied. Antibodies against R. fennica were found in wild roach
in lakes where fish are infected by the parasite. Antibody
levels were higher in sera collected in September than in sera collected
in June, due to infection of R. fennica during the
late summer. In experimental aquarium studies, roach immunized with
homogenized cercariae produced antibodies
against R. fennica. An especially strong response was elicited
by infecting fish with living cercariae emerging from infected
clams. The specificity of the antibodies, as shown in Western blots, was
different between fish immunized with homogenized cercariae and those
fish infected with living cercariae. The specificity and amount of
antibodies depended on the route of immunization. The challenge experiment
with R. fennica indicated that previous infection of fish gives
some
protection against R. fennica.