Streptococcus pyogenes Group A streptococcus, is a common human pathogen that is best known as the cause of throat and skin infections. S. pyogenes also has the potential to cause more serious and potentially life threatening diseases such as scarlet fever and toxic shock like syndrome. S. pyogenes has also been implicated as the cause of the majority of cases of puerperal fever.
The surface antigen R28 is expressed by some strains of S. pyogenes. Early studies indicated that R28 is unrelated to virulence (Lancefield and Perlmann. J. Exp. Med (1952) 96:83–97), since antibodies to R28 did not protect mice against lethal infection with an R28-expressing strain of S. pyogenes. 
Group B Streptococcus (GBS) is found in the normal flora of the human vagina and may cause life-threatening disease in newborn children who are often exposed to GBS at birth. Most isolates of GBS express either of the surface proteins Rib or a which are members of the same protein family.