Schistosoma mansoni: The role of the complement C3-activating system in the cercaricidal action of normal serum

Living Schistosoma mansoni cercariae incubated with normal serum from several species were damaged, as revealed by immobilization of their tails, uptake of methylene blue dye and the incapacity to infect appropriate vertebrate hosts. The following data indicate that the cercaricidal action of normal sera is dependent on the complement system, activation of the system proceeding through the alternate (properdin) pathway: (a) consumption of appreciable amounts of total hemolytic complement during the incubation of fresh serum with living cercariae; (b) the preferential consumption of the late reacting components and except for human C₄, only limited consumption of the classical early components; (c) the inactivity of sera depleted of C₃ or properdin; (d) the selective requirement for the presence of Mg²⁺ in the incubation mixture; (e) the full cercaricidal effect of C₄—deficient guinea pig serum; and (f) the conversion of C₃ after incubation of normal serum with the cercariae to electrophoretically faster migrating products. Immunofluorescence analyses indicated that the structures responsible for the complement activation are present in the cercarial coat.