Effect of Normal Human Serum on the Binding of Specific Antibodies and Platelet‐unrelated Immune Complexes to Human Platelets

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Abstract
Radiolabeled staphylococcal protein A was used to quantitate the binding of Ig[immunoglobulin]G on stored human platelets from human sera containing specific antibodies [Ab] reactive with platelets and rabbit serum containing immune complexes (IC). Normal human serum (NHS) inhibited the binding of IC onto platelets and to various extents the binding of specific Ab. The attachment of inhibitors to platelets seemed reversible. The considerable difference in the inhibitory capacities of IgG-deficient sera and monomeric IgG indicates that IgG is the major inhibitory component of NHS. The binding of IgG from NHS onto platelets evidently hampers the detection of weak platelet Ab even with the most sensitive tests. Purified Clq [q fragment of complement component 1] modifies the reactions of IC with fresh platelets; it did not alter the binding of IC onto stored platelets. A monoclonal, antiglobulin-active rheumatoid factor (RF) of IgM class displayed only moderate inhibition. The application of RF or Clq for the differentiation of the binding induced by IC or Ab is not useful in this assay system. The heterogeneity of immunologic receptors of platelets provides an explanation of the inhibitory inefficiency of Clq.