Measurement of Platelet Surface‐Bound IgG by a Monoclonal 125I‐Anti‐IgG Assay

We have described the use of a monoclonal 125I-labeled anti-IgG (125I-MA) to assay IgG antibody displayed on the surface of platelets from normal and immune thrombocytopenic patients and reported levels of IgG 10-100-fold lower than previous studies. This report describes the immunologic characteristics of the 125I-MA and the assay for surface IgG. The 125I-MA has a high binding affinity for surface-displayed IgG (2.22 X 10(9) M-1), reacts equally well with all four subclasses of IgG and not at all with IgM or IgA. In our assay, the binding of 125I-MA was found to be greater than or equal to 99% specific for IgG (no nonspecific association of 125I-MA with platelets) and the binding ratio of 125I-MA to IgG displayed on the cell surface was 0.91 (close to unity). Finally, platelet lysates were found to contain large amounts of IgG protein (39,597 +/- 27,418 molecules/platelet) as compared to surface-displayed IgG (124 +/- 86 molecules/platelet). This assay has excellent characteristics for quantitation of IgG on platelets and the discrepancy with other techniques may, in part, be due to intentional or inadvertent lysis of platelets during assay conditions.