Bag of pooled blood platelets. Irradiated O negative blood platelets (thrombocytes) inside a bag for transfusion. Platelets are blood cell fragments and are essential for blood clotting. These have been collected from a number of different donors (pooled), all of which have rhesus negative blood and are type O. The platelets are separated from the other blood components by centrifugation. Platelet transfusions are used to treat patients with low numbers of platelets in the blood (thrombocytopeania), or dysfunctional platelets (thrombasthenia).