Polyvalent interaction of antibodies with bacterial cells

We have studied the physical-chemical characteristics of the interaction of peroxidase-labelled rabbit antibodies with Bacillus sp. bacterial cells. The antibodies are able to bind bivalently with two antigen sites on the bacterial cells with the formation of intramolecular "cyclic" complexes. A kinetic model is proposed suggesting the existence of monovalent and bivalent cell surface antigens. The equilibrium constant of the bivalent IgG binding to the bacterial cell is by two orders of magnitude higher as compared to monovalent Fab fragments. The intramolecular reaction between the free active site of the monovalently bound antibody and a free antigen site on the cell surface is the rate limiting step of the polyvalent interaction. Formation of the cyclic complexes seems to be accompanied by essential tension of bonds and deformation of the IgG molecule. Agglutination of bacterial cells was also studied. The cell agglomerate size dependence on the antibody concn has a threshold. Agglutination proceeds under conditions where the antigen-antibody binding on the cell surface is far from equilibrium.