NETs formation in vaso-occlusive events in sickle cell disease and the role of hydroxyurea treatment.The study hypothesis is that NETs formation by neutrophils has a role in the induction of vaso occlusive disease in blood vessels. It is possible that high leukocyte count in children with sickle cell anemia is a bad prognostic sign due to NETs formation supporting occlusion of peripheral and central blood vessels.Hydroxyurea treatment might prevent vaso occlusive syndrome not only by increasing HbF but also by decreasing neutrophil count and inhibiting NETs formation.