Electron microscopic study of the location of the Fc-reacting factor on group A streptococci

The presence of the Fc-reacting factor was demonstrated on four out of five different group A Streptococcus strains using ferritin-labelled immunoglobulin G. A comparison of the results obtained by this electron microscopic technique with Fc-reacting factor detection results obtained with hydrochloric acid extracts of cells in passive haemagglutination on sensitized red cells, showed that not only hydrochloric acid extractable but also non-extractable Fc-reacting factor can be present on the group A Streptococcus cell. Out of several IgGs tested, only rabbit and swine IgGs bound to Fc receptors of group A Streptococcus walls. The Fc-reacting factor is ultrastructurally localized on the tips of the filamentous protrusions forming the outermost layer of the Streptococcus wall. The involvement of the Fc portion of IgG in the reaction was demonstrated by a positively reacting sandwich arrangement in which Streptococcus cells were incubated with rabbit antiferritin before being treated with ferritin.