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Abstract In previous studies on the interaction of pneumococci and the heat labile opsonin (HLO) system it was found that C3 was fixed to the surface of the pneumococcus and that this reaction resulted in opsonization of the organism. The activation of C3 was demonstrated to proceed via the properdin pathway and IgG2, isolated from “non-immune” guinea pig serum (GPS), was shown to participate functionally in the C3 activation. The present studies concern the role of IgG2 in the interaction of pneumococci with the properdin pathway. The functional activity present in the IgG2 globulin fraction of normal GPS is a 7S immunoglobulin. It is completely removed after passage through an immunoabsorbent column of rabbit-anti-GP IgG1 and IgG2 and is concordant with a 7S peak on sucrose density gradient ultracentrifugation. The Fc portion of the IgG2 is not essential for its activity since after pepsin digestion the activity is retained. The IgG2 possesses some degree of specificity for the pneumococcus since immunospecific GP IgG2-anti-DNP will not function in the interaction of pneumococci and the properdin pathway. However, both pneumococci and Escherichia coli are able to absorb out the functionally active IgG2, indicating that the immunoglobulin binds to some common surface antigen shared by both organisms.
Winkelstein et al. (1974) studied this question.