Type-specific antibody to the opacity factor (OF) of group-A streptococci can be demonstrated in human sera but the multiplicity of antibodies to different serotypes limits their usefulness in anti-OF typing. The antibody response in rabbits is inconsistent; only 61 of 138 (44%) of rabbit anti-M sera tested contained OF antibody. Of these only about half had titres of > 16, and usable sera to only 11 of 23 OF-positive serotypes were obtained. On the other hand good anti-OF sera (titres > 16) to 27 of 28 serotypes resulted from the interperitoneal and subcutaneous inoculation of heat-killed whole-cell vaccines in guinea-pigs and the frequency of response in groups of animals given injection of the same vaccine was 100% for all but three serotypes. Antibody response was not obtained with M-type 13. A comparison of routes of inoculation for M-type 25 showed that the subcutaneous route alone could probably be used in the routine production of anti-OF typing sera. Use of the set of 27 sera in OF-inhibition tests confirmed the remarkable specificity of OF antigens and their parallelism with M-antigen specificity, with the exception of a reciprocal cross reaction between M-type 61 and provisional type PT3875.


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