All of the following are characteristics of streptococcal M proteins except

• A. antibodies to one serotype protect against other serotypes.
• B. it provokes an immune response.
• C. it limits phagocytosis of the organism.
• D. it is the chief virulence factor of Group A streptococci.

Answer: (C)

Streptococcal M protein is a surface protein that acts as a major virulence determinant in Group A Streptococcus and is highly antigenic, triggering a host immune response. It helps the bacterium resist phagocytosis by interfering with opsonization and complement deposition, making it a key factor in the organism’s ability to cause disease. However, because there are many distinct M protein serotypes, antibodies raised against one serotype generally do not protect against other serotypes; this lack of cross-protection is the aspect that does not fit with the other true features. The remaining statements—that it provokes an immune response and that it limits phagocytosis—are accurate, and it is considered a major virulence factor for Group A streptococci.