Specific but nonimmunologic reaction between staphylococcal protein A and the Fc portion of gamma globulin provided the basis for ultrastructural studies to determine the localization of protein A, using intact staphylococci and labeled myeloma gamma G-globulin.

Protein A appeared to be part of the outermost layer of the staphylococcal cell wall. Strains with protein A demonstrated a coating of myeloma globulin over the entire bacterial surface. There was no coating of strains without protein A.

Identification of protein A on the surface of the staphylococcal cell wall provides evidence that this may be the first material in contact with host environment. It probably accounts for apparent cross-reactions of staphylococci with antibodies to many antigens. More importantly, even in the nonimmune host protein A immunoglobulin reactivity may initiate complement activation and inflammatory reactions including chemotaxis and pus formation.

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