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 Fab'2 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'2 epsilon fragment but not the Fc" epsilon fragment retained the ability to react with protein A. IgG preparations from different species were tested for ther 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'2 gamma fragments, but not by Fc gamma fragments, suggests a novel type of interaction of protein A with immunoglobulins.