Correct Option: B

Protein A is a cell wall component produced by several strains of Staphylococcus aureus that consists of a single polypeptide chain and contains little or no carbohydrate. The Protein A molecule contains four high-affinity binding sites capable of interacting with the Fc region from IgG of several species including human and rabbit. Optimal binding occurs at pH 8.2, although binding is also effective at neutral or physiological conditions (pH 7.0 to 7.6). The interaction between Protein A and IgG is not equivalent for all species. Even within a species, Protein A interacts with some subclasses of IgG and not others. For instance, human IgG1, IgG2 and IgG4 bind strongly, while IgG3 does not bind. There are also many instances in which monoclonal antibodies do not bind to Protein A, especially the majority of rat immunoglobulins and mouse IgG1.