Figure 1.

Unwanted passengers and events during platelet storage: Besides platelets, platelet concentrates contain “unwanted passengers” in the form of soluble mediators in the supernatant (in the case of unwashed platelet concentrates – which are still the routine product in virtually all US hospitals) and leukocytes (in the case of non-leukoreduced platelet concentrates – which are still an available product in some US hospitals). The supernatant (red background) contains substances which are secreted or shed into the plasma during storage containing soluble mediators, like sCD40L, PDGF, or sP-selectin. In addition, cytokines and cell fragments can be found. The platelet themselves (yellow background) change during storage. They become hyporesponsive (not depicted), and degranulate with increased expression of alpha granule contents (e.g., P-selectin). Platelets express ABO antigens, which lead to suboptimal results and complications if transfused into ABO-unmatched recipients. A marker for platelet activation is platelet leukocyte aggregate formation (white background). With prolonged storage, the number of platelet leukocyte aggregates increase. Leukocytes in the stored bag (gray background) release cytokines, which can induce febrile transfusion reactions. Passenger leukocytes lead to sensitization to HLA antigens with subsequent refractoriness.