The binding of complement by complexes formed between a rabbit antibody and oligosaccharides of increasing size

Immune complexes formed between a homogeneous rabbit antibody to type III pneumococcal polysaccharide and a series of oligosaccharides of varying size derived from it were prepared and tested for their ability to fix guinea pig hemolytic complement. Antibody and either tetra-, hexa-, or octasaccharide formed only monomeric antibody-hapten complexes and did not show any complement binding. A dodecasaccharide and a 16-sugar residues oligomer formed dimer and trimer immune complexes. These complexes were also unable to fix complement. However, as the size of the sugar oligomers was increased to about 21 sugar residues per oligosaccharide molecule or more, the resulting complexes exhibited substantial complement binding, concomitant with the formation of antigen-antibody aggregates higher than trimers. On the other hand, an independent study carried out with the same material suggested changes in the conformation of the Fc moiety in the antibody molecule upon addition of oligosaccharide ligands as small as a 16-residue unit. Since the resulting complexes hardly ehibited any complement binding, ligand-induced conformational changes in the Fc part of the antibody molecule appears to be an insufficient condition per se for triggering complement fixation.