In patients with aTTP, levels of a substance called von Willebrand factor are increased. von Willebrand factor acts on platelets to cause them to stick together and form blood clots. Caplacizumab, the active substance in Cablivi, is a nanobody (a small antibody) which has been designed to attach to von Willebrand factor in a way that stops it acting on platelets. This reduces platelets sticking together and forming clots in blood vessels and, as a result, platelet levels in the blood rise because they are no longer taken up to form clots.