Detection of platelet-bound IgG with 125I-labelled staphylococcal protein A.

125I-labelled protein A from Staphylococcus aureus binds effectively with platelets coated either with specific antibodies or with unrelated immune complexes. Protein A can therefore be used to measure these serum factors. A prior dilution of the serum enhances markedly the reactivity of the serum in idopathic thrombocytopenic purpura. This may be due to abolished inhibition exerted by normal monomeric IgG.