C3 levels in the sera of rabbits infested and reinfested with Ixodes ricinus L. and in midguts of fed ticks.

Serum C3 levels of rabbits infested 3 times with Ixodes ricinus L. females and C3 in midgut extracts of fed ticks have been measured by a single radial immunodiffusion test. From the first tick attachment, the mean serum C3 level of hosts increased. A peak of C3 occurred 6 days after the beginning of each infestation and was highest during the third infestation (about 8 times more pronounced than the level measured before the first). After the end of all infestations of the rabbits, the serum C3 level decreased and approached normal levels. Modifications of serum C3 levels during reinfestations influenced C3 contents in the blood meal of fed ticks. Midguts of ticks fed during the third infestation contained more C3 than midguts of ticks of the same engorged weights fed during the first or second infestations.