Invertebrates lack adaptive immune systems homologous to those of vertebrates, yet it is becoming increasingly clear that they can produce diversified antigen recognition molecules. We have previously noted that the snail
Biomphalaria glabrata
produces a secreted lectin, fibrinogen-related protein 3 (FREP3), unusual among invertebrate defense molecules because it is somatically diversified by gene conversion and point mutation. Here we implicate FREP3 in playing a central role in resistance to a major group of snail pathogens, digenetic trematodes. FREP3 is up-regulated in three models of resistance of
B. glabrata
to infection with
Schistosoma mansoni
or
Echinostoma paraensei
, and functions as an opsonin favoring phagocytosis by hemocytes. Knock-down of FREP3 in resistant snails using siRNA-mediated interference resulted in increased susceptibility to
E. paraensei
, providing a direct link between a gastropod immune molecule and resistance to trematodes. FREP3 up-regulation is also associated with heightened responsiveness following priming with attenuated digenetic trematodes (acquired resistance) in this model invertebrate immune system.
