Platelet IgG: measurement, interpretation, and clinical significance

Platelet IgG has been studied primarily to develop an aid for clinical diagnosis--a search that has proven elusive (48). However, studies of the interaction of IgG and other plasma proteins with platelets have provided new insight into the intracellular pathways for the acquisition of proteins into regulated secretory granules. From a clinical perspective, it is important not to make the assumption that all platelet IgG is antiplatelet antibody. Data on platelet IgG must be distinguished between measurements of alpha-granule IgG, which comprises almost all of the total platelet IgG content, and platelet surface IgG, representing less than 1 percent of total platelet IgG. alpha-granule IgG does not appear to be antiplatelet antibody; platelet surface IgG is likely to be at least partly antiplatelet antibody. The total platelet (alpha-granule) content of IgG mirrors the concentration and composition of plasma IgG. It is increased in patients with hypergammaglobulinemia of any cause and absent in agammaglobulinemia. The platelet alpha-granule content of IgG, IgA, and albumin is proportional to their concentrations in plasma, and the platelet content of IgG, IgA and albumin also reflects their plasma concentration over a wide range of plasma abnormalities. These observations suggest that platelets acquire IgG, IgA, and albumin by fluid phase endocytosis. Total platelet IgG is consistently increased in patients with ITP and is also commonly increased in patients with thrombocytopenia due to increased platelet destruction of apparently nonimmune mechanisms. The total platelet IgG content does not appear to be increased in patients who have thrombocytopenia due to marrow failure. Therefore, the measurement of total platelet IgG may be analogous to the reticulocyte count, which is increased in patients who have accelerated erythropoiesis caused by either immune or nonimmune hemolytic anemia. The increased concentration of total platelet IgG in patients with ITP is parallel to the appearance of larger platelets produced during the stress of increased thrombopoietic stimulation. Therefore, determinations of mean platelet volume may yield clinical information equivalent to the measurement of total platelet IgG. The concentration of platelet surface IgG is greatly increased in patients with ITP, and concentrations are highest in patients with the most acute ITP. Platelet surface IgG may represent antiplatelet antibody in these patients. However, for unknown reasons, modest elevations of platelet surface IgG are found in many patients with nonimmune thrombocytopenia.(ABSTRACT TRUNCATED AT 400 WORDS)