On the composition of IgG anti-IgG immune complexes

The formation of different immune complexes of IgG was followed in a system comprising human IgG as antigen and rabbit IgG directed against the Fc portion of human IgG as antibody. Soluble IgG complexes were analyzed by analytical zonal centrifugation. In antigen excess, 16S complexes predominated. 16S complexes are oligomers of IgG, mainly trimers and tetramers. In decreasing antigen excess larger and larger complexes were formed. It was, however, found consistently that oligomers were always formed in the largest amounts. The largest complexes detectable by this method consisted of about twenty IgG molecules. The solubility of different complexes in polyethylene glycol was also studied. Low concentrations of polyethylene glycol preferentially precipitate large complexes. Four and six per cent polyethylene glycol precipitated all types of IgG complexes although not completely. Polyethylene glycol was seemingly not bound directly to soluble immune complexes, but caued otherwise soluble complexes to precipitate by an indirect mechanism.