New immune systems: Pathogen-specific host defence, life history strategies and hypervariable immune-response genes of invertebrates

Our understanding of invertebrate immune systems is undergoing a paradigm shift. Until recently, the host defence responses of invertebrates were thought to rely on limited molecular diversity that could not tailor reactions toward specific microbes. This view is now being challenged. Highly discriminatory defence responses, and hypervariable gene systems with the potential to drive them, have been identified in a number of invertebrate groups. These systems seem to be quite distinct, suggesting that pathogen-specific responses might have evolved on numerous occasions. Here, we review evidence that inducible, disease-specific immunity might be commonplace in the animal kingdom.