no more than three units of platelets are generally harvested in any one sitting from a donor. Most donors will donate a "single" or "double" unit, however the occurrence of "triples" has been increasing as more suitable donors are recruited. Because platelets have a shelf life of just five days, more platelet donors are always needed. Some centers are experimenting with seven-day platelets, but this requires additional testing and the lack of any preservative solutions means that the product is far more effective when fresh. Even though red blood cells can also be collected in the process, many blood donation organizations do not do so because it takes much longer for the human body to replenish their loss. In addition, some (though not all) centers defer further platelet donations until the red blood cells can be replenished. In most cases, blood plasma is returned to the donor as well. However, in locations that have plasma processing facilities, a part of the donor's plasma can also be collected in a separate blood bag (see plasmapheresis). For example, in Australia around 5.9×1011 platelets and 580ml of plasma might be collected from an 88kg donor. Leukoreduction Due to their higher relative density, white blood cells are collected as an unwanted component with the platelets. Since it takes up to 3 liters of whole blood (the amount of a dozen blood bags) to generate a dose of platelets, white blood cells from one or several donors will also be collected along with the platelets. A 70 kg (154 lb) man has only about 6 liters of blood. If all of the incidentally collected white blood cells are transfused with the platelets, substantial rejection problems can occur. Therefore, it is standard practice to filter out white blood cells before transfusion by the process of leukoreduction. Early platelet transfusions used a filter to remove white blood cells at the time of transfusion. It takes a trained person about 10 minutes to assemble the equipment, and this is not the safest or most efficient means of filtration because living white blood cells can release cytokines during storage and dead white blood cells can break up into smaller fragments that can still stimulate a dangerous response from the immune system. In addition, simple filtration can lead to increased risks of infection and loss of valuable platelets. Newer, more advanced thrombapheresis machines can filter white blood cells during separation.