Wastewater treatment plant effluents from urban area are a well-known source of chronic multiple micropollution to the downstream living organisms. In this study, ecologically relevant laboratory-bred freshwater gastropods, Lymnaea stagnalis, were exposed for 29 days to raw effluents of a wastewater treatment plant in Lyon area (France). A time-course analysis of individual markers of immunocompetence (hemocyte density and viability, hemocyte NADPH activity, phenol oxidase activity, and capacity of phagocytosis) has shown slight trends of inflammatory-like responses induced by the 100% effluents. So far, no short-term hazard for L. stagnalis can be revealed. However, over the long term, such environmental stress-stimulating immune responses could provoke deleterious life history trade-offs because the immune system is known to be highly energy-consuming.