Protective immunity to the group A Streptococcus may be only strain specific.

M protein enables Group A streptococci to resist phagocytosis. Protective immunity is considered to be mediated by opsonic antibodies directed against this M protein. In recent studies we have shown that genetically distinct populations exist within an M-type. The question asked in this study was whether human and rabbit type specific M1 antibody was opsonic for all strains of M-type 1, irrespective of their restriction fragment length polymorphism type. When locating a blood donor from among our staff for use in the indirect bactericidal test, selective variation in opsonic ability was demonstrated by one person. Subsequent testing of 44 randomly selected human sera revealed that 11 (25%) had opsonic antibody. Of these 11, 6 opsonised all seven strains and 5 demonstrated selective opsonisation. We conclude that opsonic antibody is not necessarily type specific but may be strain specific.