The amount of IgG on the surface of washed platelets from healthy individuals varies according to the assay used. An immunoradiometric assay (IRMA) for IgG was developed, validated and used to measure platelet associated IgG. The platelet-associated IgG (PAIgG) on washed platelets was allowed to react with excess125I-labelled anti-IgG. The unbound I-anti-IgG was then quantitated by the addition of sepharose beads to to which IgG had been covalently bound. The125I-anti-IgG/IgG-beads were separated from the platelet suspension by passage across a density gradient. The mean amount of IgG present on washed platelets from healthy individuals was 0.8 fg IgG/platelet, or approximately 4,000 molecules of IgG per cell. Inverse Scatchard analysis confirmed the validity of the assay calibration and identical results were obtained when five different anti-IgG antibodies were used. The specific binding of I-anti-IgG to platelet-bound IgG was complete by 30 min, but non-specific binding of radioactivity continued thereafter. This non-specific binding occurred not only with anti-IgG but with all antibodies tested and could give elevated estimates of PAIgG in direct binding assays.
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