Platelets as evolution's answer to both hemorrhage and infection

Hemorrhage and infections have remained substantial causes of mortality and morbidity throughout history. Platelets are classically known for their role in thrombosis. However, our recent understanding has revealed their role in innate immunity. This historical sketch explores the evolutionary origins of platelets through the concept of immunothrombosis. We trace the emergence of hemocytes in invertebrates, the development of thrombocytes in early vertebrates, and the eventual rise of anucleate platelets in mammals. Mammalian platelet fragments retain immune capabilities alongside their clotting abilities. These include pathogen sensing, antimicrobial peptide release, and coordination with leukocytes. Evolutionary pressures may have shaped blood-based systems that rapidly respond to both vascular injury and microbial invasion. We argue that platelets represent an elegant solution to the evolution's threats of hemorrhage and infection.