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 Mprotein
following GAS infection, but M-proteins are diverse and subject
to mutation, so immunity to one M-subtype may not confer immunity to
others.