Approximately 50 per cent of Group A, Type 3 streptococci contain a hitherto undescribed antigen found only in Group A, Type 3 organisms. It is serologically distinct from Type 3 M antigen and is designated as 3 R antigen.

Strains containing 3 R antigen but no Type 3 M antigen are "glossy," avirulent Type 3 variants. These strains can be obtained by repeated transfers of virulent M-containing streptococci in artificial media under unfavorable conditions of growth. These degraded streptococci recover Type 3 M antigen during serial passage through mice. The amount of 3 R antigen in a strain is not affected by a decrease or increase in M antigen.

The 3 R antigen is unrelated to virulence. Antibodies to this antigen do not protect mice against infection or promote phagocytosis in bactericidal tests. The 3 R antigen-antibody system can give rise to confusion in M-precipitin reactions. In all these properties 3 R antigen is similar to 28 R antigen, although in certain other properties the two R antigens are not identical. They are serologically distinct.

Virulence and ability to grow in normal human blood under the conditions of the bactericidal test are correlated with the presence of M antigen of Group A, Type 3 streptococci. Mouse protection and specific inhibition of growth of Type 3 streptococci by phagocytosis in bactericidal tests are associated with the presence of Type 3 anti-M antibodies.

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