The separation of individual blood components is done with a specialized centrifuge (see apheresis). The earliest manual forms of thrombapheresis are done by the separation of platelets from multiple bags of whole blood collected from donors or blood sellers. Since each blood bag (usually 250 mL or 500 mL) contains a relatively small number of platelets, it can take as many as a dozen blood bags (usually from 5 to 10 bags, depending on the size of the blood bags and each donor's platelet count) to accumulate a single unit of platelets (enough for one patient). This greatly increases the risks of the transfusion. Each unit of platelets separated from donated whole blood is called a "platelet concentrate".