Normal human immunoglobulin G4 is bispecific: it has two different antigen-combining sites

Unlike other immunoglobulin G (IgG) subclasses, IgG4 antibodies in plasma h ave been reported to be functionally monovalent. In this paper we show that the apparent monovalency of circulating IgG4 is caused by asymmetry of pla sma IgG4. A large fraction of plasma IgG4 molecules have two different anti gen-binding sites, resulting in bispecificity. Sera from patients with IgG4 antibodies to both house dust mite and grass pollen induced cross-linking of Sepharose-bound grass pollen antigen to radiolabelled house dust mite al lergen Der p I. This bispecific binding activity was not observed in sera w ith IgG4 antibodies to either grass pollen or house dust mite exclusively. Depletion of IgG4 antibodies resulted in disappearance of the bispecific ac tivity. By size exclusion chromatography we excluded the possibility that b ispecific activity was caused by aggregation of IgG4 antibodies. These resu lts indicate that circulating (polyclonal) IgG4 antibodies have two differe nt antigen-binding sites and therefore are functionally monovalent antibodi es.