The surfaces of the parasitic nematodes Trichinella spiralis and Toxocara canis differ in the binding of post-C3 components of human complement by the alternative pathway

The binding of human complement components C3, C5 and C9 to the surface of the infective larvae of the nematode parasites Toxocara canis and Trichinella spiralis, by the alternative pathway, was examined by direct and indirect immunofluorescence on the intact parasites. This showed that although C3 bound to both nematodes, they differed markedly in the binding of C5 and C9; C5 bound only minimally to T. spiralis, and C9 binding to this parasite was barely detectable. In contrast, both early and late components bound to T. canis to a high density, comparable to, or in excess of, the binding of these components to the infective larvae of the trematode Schistosoma mansoni. The lack of binding of the post-C3 components to T. spiralis did not correlate with enhanced binding of the control protein, Factor H.