IgG F(ab‘)2 OF RABBIT ANTI‐M SERA BUT NOT THE UNFRACTIONATED SERA ARE BACTERICIDAL FOR SOME GROUP A STREPTOCOCCI WITH IgG Fc‐RECEPTOR ACTIVITY: Opsonic Effect Ascribable to Anti‐IgG

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Abstract
We have earlier reported on a group A streptococcal strain, type M12, which upon serial mouse passage acquired IgG Fc‐receptor activity but lost the M‐antigen. The passaged strain, 12P, was highly virulent for mice and grew well in normal human blood. The present study particularly concerns the opsonic effect on 12P of rabbit anti‐M3, anti‐M 12 and anti‐12P sera, as well as the corresponding IgG F(ab')2‐ Indirect bactericidal tests showed that the homologous anti‐12P serum and IgG F(ab')2 were opsonic. The anti‐M3 and anti‐M 12 had no effect on 12P; surprisingly, however, IgG F(ab')2 isolated from these sera displayed a clearcut opsonic activity. Data are presented which indicate that these »paradoxical« results can be explained by the binding of IgG F(ab')2 with anti‐IgG specificity to human IgG, linked to the streptococcal surface through Fc‐receptors. Only anti‐12P serum, or IgG F(ab')2, were protective for mice on challenge with strain 12P.

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