Protein A reactivity of whole rabbit IgG and of fragments of rabbit IgG.

Approximately 15 per cent of the total IgG in normal rabbit serum reacted with protein A. When this protein reactive IgG fraction was treated with acid at pH 2.7 and then digested with plasmin, thus splitting the rabbit IgG between the CH2 and CH3 domains, both the isolated Facb and the pFc' fragments were protein A non-reactive. This results indicates that at least one intact Fc chain is necessary to fulfil the protein A-fc interaction. Traces of protein A reactive rabbit F(ab')2 fragments isolated from normal IgG were also found. These fragments mixed with normal rabbit IgG precipitated protein A.