Immunochemical Study of the Interaction between Staphylococcal Protein A, Rabbit Antistaphylococcal Sera, and Selecred Sera from Nonimmunized Animals

This study is a further investigation of the interaction between staphylococcal protein A and mammalian IgG, particularly as regards the three‐component immunoprecipitation that was described by Kronvall and Williams in 1971 and designated the ‘star phenomenon’. For IgG or the F(ab')2 fragment of IgG, the ability to precipitate protein A did not correlate with the occurrence of staphylococcal antibodies, as demonstrated in an indirect hemagglutination test. or with antibodies that inhibit α‐hemolysin. The star phenomenon required the following three components: protein A; IgG forming soluble complexes with protein A; and F(ab')2 fragments obtained either from rabbit antisera to protein A, from normal IgG that precipitated protein A, or from human myeloma IgG that precipitated protein A. These different (F(ab')2 fragments could co‐operate in the formation of a star only if both protein A and an intact IgG were present. It is suggested that star‐forming F(ab')2 fragments possess combining sites directed against hidden determinants on the IgG molecule, which are made accessible when protein A binds to the Fc piece.