Coagulation systems of invertebrates and vertebrates and their roles in innate immunity: the same side of two coins?

Bacterial infections represent a serious health care problem, and all multicellular organisms have developed defense mechanisms to eliminate pathogens that enter the host via different paths including wounds. Many invertebrates have an open circulatory system, and effective coagulation systems are in place to ensure fast and efficient closure of wounds. It was proposed early on that coagulation systems in invertebrates play a major role not only in sealing wounds but also in preventing systemic infections. More recent evidence suggests that vertebrates, too, rely on clotting as an immune effector mechanism. Here we discuss the evolution of clotting systems against the background of their versatile function in innate immunity.