Platelets: much more than bricks in a breached wall.

Platelets have various roles in vascular biology and homeostasis. They are the first actor in primary haemostasis and play important roles in thrombosis pathogenesis, but they are also part of innate immunity, which initiates and accelerate many inflammatory conditions. In some contexts, their immune functions are protective, while in others they contribute to adverse inflammatory outcomes. Platelets express numerous receptors and contain hundreds of secretory molecules that are crucial for platelet functional responses. The capacity of platelets to produce and secrete cytokines, chemokines and related molecules, under the control of specific intracellular pathways, is intimately related to their key role in inflammation. They are also able to intervene in tissue regeneration and repair because they produce pro-angiogenic mediators. Due to this characteristic platelets are involved in cancer progression and spreading. In this review we discuss the complex role of platelets, which bridges haemostasis, inflammation and immune response both in physiological and pathological conditions.