Platelets in leucocyte recruitment and function

Platelets have a longstanding recognition as an essential cellular component of the coagulation system. However, substantial research over the last decade has added another important aspect to platelet function in that they are also an integral part of the innate immune system. Complex organisms are facing a constant threat of infections by invading pathogens, and they have developed a sophisticated and elegant measure to combat this threat, namely the immune system. Leucocyte recruitment to sites of infections is an essential step at the forefront of the immune response. Platelets have been shown to be involved in several steps of this process and they are an integrated connecting element among haemostasis, host defence, and additional immunological functions (e.g. neutrophil extracellular traps formation). However, the immune system also requires a tight regulation, as an overshooting immune response carries the risk of harming the host itself. This review aims at highlighting the unique features and molecular mechanisms that allow for the interactions of platelets and leucocytes and the regulation of this process. Furthermore, this article identifies the functional relevance of these events for the immune response.