Blood platelets are circulating stores for adhesive proteins, inflammatory mediators, and immunoglobulins - Role in nonhemolytic transfusion reactions

Objective: Blood platelets store within their α-granules a variety of substances that are involved in blood clotting, inflammation, and repair processes after a lesion of the vessel wall. Not all of these substances are synthesized within the megakaryocytes or platelets themselves, but some of them are taken up from the blood plasma by the circulating platelet. This article summarizes recent investigations on the internalization capabilities of platelets and the mechanism of endocytosis. Results: By means of immunoelectron microscopy and by incubations with gold-labeled peptides the uptake of fibrinogen, IgG, IgE, and the chemokine RANTES by the platelet can be demonstrated. Two different mechanisms of internalization are discerned: In resting platelets, the gold-labeled peptides are endocytosed and transferred to α-granules, whereas in platelets stimulated by ADP or thrombin the gold particles appear within the open canalicular system and not within granules. Conclusion: Platelets store and release not only proinflammatory mediators that are typical for the megakaryocyte-platelet lineage, but also blood-borne substances from other cellular sources. Due to activation and disintegration of platelets that are stored as concentrates for usage in transfusion medicine, these substances can accumulate in the platelet concentrate and might be responsible for transfusion-associated reactions. Further investigation in this field of platelet immunobiology is warranted.