Effects of Staphylococcal Protein a on Heat Labile Opsonins

Staphylococcal protein A inhibited phagocytosis of Staphylococcus aureus and Escherichia coli in in vitro phagocytosis systems with normal human serum. Protein A had no effect on phagocytic cells per se, and produced inhibition of the heat labile opsonins, i.e., of the complement system in normal sera at low concentration. At higher concentration of protein A there was little complement fixation and little inhibition of serum opsonic activity.