All animals are plagued by parasites of some variety, but parasites all have one thing in common – profiteering at their host's expense. Naturally, hosts aren't too happy about this and will engage their immune systems to fight back. However, the outcome of the ensuing battle can also be affected by ecological and environmental factors. These factors can then modify the host–parasite relationship, causing changes in host's life choices as well as driving evolution. However, this growing field of ecoimmunology – immunology in the context of ecology – has been hampered by limited knowledge of how different immune elicitors activate the immune system. Without this knowledge, knowing which elicitor to use for your specific experiment can be tricky, so Otto Seppälä and Katja Leicht both from Eawag, Switzerland, decided to investigate how the freshwater snail Lymnaea stagnalis reacted to different threats (p. 2902).
The team injected their snails with two different types of lyophilized bacteria, Escherichia coli and Microccocus lysodeikticus, as well as ground-up gonads from an unlucky snail infected with a trematode parasite. Some snails escaped these treatments and just received innocuous injections of snail saline or ground-up uninfected gonads. Six hours post-treatment, the duo collected haemolymph samples and began testing the immune reaction, looking at phenoloxidase and antimicrobial activity. Merely pricking the snail and injecting harmless snail saline induced phenoloxidase activity but to their surprise injection with trematode-infected gonads didn't increase this reaction. What's more, this activity decreased when the snails were injected with bacteria. This may represent a trade-off between phenoloxidase activity and antimicrobial activity, as this latter immune response was heightened after Microccous infection.
Next, the duo kept their snails in either clean water or bacteria-ridden water and repeated the same measurements of the immune system. This time, the presence of micro-organisms induced a strong phenoloxidase response but no antimicrobial activity. Their results overall highlight the complexity of the snail's immune system, with trade-offs occurring between different pathways. It seems that the immune system of the snail remains a mystery!