Ans. is 'b' i.e., Cell wall Protein A is a cell wall component of many S aureus strains that binds to the Fc poion of IgG molecules except IgG3.The Fab poion of IgG bound to protein A is free to combine with a specific anigen. Protein a has become an impoant reagent in immunology and diagnostic laboratory technology; for example, protein A with attached lgG moleculaes directed against a specific bacterial antigen will agglutinate bacteria that have that antigen ("coagglutination").