B domain of Staphylococcus aureus protein A, molecular model. Protein A is a surface factor found on the membrane of S. aureus bacteria. The B domain, shown here, binds to immunoglobulin (antibodies) in the blood so as to prevent the bacteria from triggering an immune response. In this model, the solid colours represent the protein's secondary structure (spiral ribbons are alpha helices); the faded tubular structure behind represents the individual atoms and bonds.