Significance of anti-IgG antibodies obtained by immunization of rabbits with some streptococcal strains.

Anti-human IgG antibodies have been found in a number of rabbit sera immunized with freshly isolated streptococci. The significance of these anti-IgG antibodies was investigated. An IgM rheumatoid factor was excluded because the antibodies to human IgG present in rabbit sera belonged to the IgG class and reacted with the different isotypes of the human 7 S IgG. No cross-reaction between streptococci and human IgG was found. The anti-IgG production could be attributed to the in vivo non-immunological binding of host IgG to the infecting streptococci, via their Fc(gamma) receptors. A strong adjuvant effect of streptococci for IgG was found, and it explained the noticeable induction of anti-IgG antibodies by trace amounts of immunogen bound to streptococci. The possible induction of anti-human IgG antibodies by streptococcal cells could be an alternative explanation of the numerous cross-reactions described between streptococci and different human tissues.