Galactose-containing epitopes on the surface of IgG model immune complexes are accessible for specific binding with the high molecular weight ligand, Ricinus agglutinin, in solution-light-scattering studies.

Immunoglobulin G (IgG) molecules contain covalently linked carbohydrate chains with galactose residues in their branched "antennae". The ability of galactose-containing epitopes on the surface of IgG model immune complexes (IC) to interact with a high mol. wt ligand in solution has been elucidated. Different types of IgG model IC with pre-determined molecular mass were mixed with Ricinus Agglutinin (RCI), which is known to bind specifically to galactose-containing oligosaccharides. The relative light-scattering increases (delta I) in the reaction mixture were measured as a function of time. The galactose-associated epitopes of the IgG model IC were accessible for binding with RC1. The rate of the interaction between IgG model IC and RC1 was dependent on the molecular mass of the complexes; the larger the model IC molecular mass, the faster the rate of interaction. The binding of RC1 to IgG model IC was highly specific because it was completely abolished in the presence of lactose. The galactose-containing epitopes of monomeric IgG were also able to interact with RC1 but the kinetics of the interaction was much slower. We suggest than an increase in the density of the epitopes on the surface of the model IC, by close attachment of the IgG molecules, mainly determines the ability of galactose-containing epitopes to be recognized by RC1. The data presented support the importance of IgG glycans in recognition events of IgG by biologically active molecules.