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Interaction of Human Polyclonal IgE and IgG from Different Species with Protein A from Staphylococcus aureus: Demonstration of Protein‐A‐reactive Sites Located in the Fab2 Fragment of Human IgG

The peptic fragments F(ab′)2 and Fc″ of human polyclonal IgE were tested by the SpA‐IgE radioimmunoassay for interaction with protein A from Staphylococcus aureus. The Fab″2ɛ fragment but nut the Fc″ɛ fragment retained the ability to react with protein A. IgG preparations from different species were tested for their capacity to inhibit the reaction between 125I‐IgE and protein A. IgG preparations from man and the dog, pig, guinea‐pig and rhesus monkey were all potent inhibitors whereas those from the rabbit, rat, sheep, horse, cow, goat and mouse were very poor inhibitors. The inhibition of the binding of 125I‐IgE to protein A by Fab′2γ fragments, but not by Feγ fragments, suggests a novel type of interaction of protein A with immunoglobulins.