For example, with marginally acceptable whole blood (white blood cells: < 10,000/mm³; platelets: > 150,000/mm³), a dose (3×1011) of platelets comes with about 2×1010 white blood cells. This can seriously damage the patient's health. A dose of single-donor platelets prepared using latest filters can contain as little as 5×106 white blood cells. Apheresis There are two types of manual platelet apheresis. Platelet-rich plasma (PRP) is widely used in North America and buffy coat (BC) is more widely used in Europe. Platelets are the clotting cells of the blood, and when donated, frequently go to cancer patients, because due to chemotherapy many cancer patients are unable to generate enough platelets of their own. The basic principles of automatic platelet apheresis are the same as in the manual procedure, but the whole procedure is performed by a computer-controlled machine. Since the donor's blood is processed in a sterile single-use centrifuge, the unwanted components can be returned to the donor safely. This allows the apheresis machine to repeat the draw-centrifuge-return cycle to obtain more platelets. The bulk of the machine and the length of the donation process means most platelet donations are done in blood centers instead of mobile blood drives. Each country has its own rules to protect the safety of both donor and recipient. In a typical set of rules, a platelet donor must weigh at least 50 kg (110 lb) and have a platelet count of at least 150 x 109/L (150,000 platelets per mm³). One unit has greater than 3×1011 platelets. Therefore, it takes 2 liters of blood having a platelet count of 150,000/mm³ to produce one unit of platelets. Some regular donors have higher platelet counts (over 300,000/mm³); for those donors, it only takes about one liter of their blood to produce a unit. Since the machine used to perform the procedure uses suction to draw blood out of a donor's body, some people who can give whole blood may have veins too small for platelet donation. Blood centers evaluate each donor's veins prior to donation. Blood accounts for about 8% of body weight, so a 50 kg (110 lb) donor has about four liters of blood. No more than 50% of a donor's platelets are ever extracted in one sitting, and they can be replenished by the body in about three days. Most newer apheresis machines can separate a maximum donation of platelets in about 60 to