The M-protein is a key virulence factor in GAS infection. Virulence factors are those parts of a microorganism that aid the infection process and help the bacterium evade host immune responses. M-protein enhances bacterial attachment and invasion in the pharynx and inhibits phagocytosis by immune cells. A person develops antibodies to the M-protein following GAS infection, but M-proteins are diverse and subject to mutation, so immunity to one M-subtype may not confer immunity to others. (9)