Bivalent epitope binding

These studies are the only structural studies to suggest that antibody binding may affect the capsid in ways other than just small, localized changes at the epitope—paratope interface. However, these changes are not related to bivalent binding of the antibody, nor are they related to neutralization efficacy. The conclusion from these studies is, again, that the major effect of the antibody on viral infectivity is steric abrogation of receptor-virus interactions. 

Avidity represents the strength of all binding events that occur in an antibody antigen interaction. It is not simply the sum of all affinities in a PAb antibody preparation, it also depends on the valencies of the antibody and antigen. IgG is a bivalent molecule, whereas IgM is decavalent, and the antigen can be multivalent as well if there is a multiplication of epitope within the molecule or the presence of multiple epitopes recognized by different antibodies. Conformational changes to... 

Figure 5 shows that there must be two identical antigen-binding sites (more in the case of serum IgM and secretory IgA) hence, the basic Y-shaped Ig molecule is bivalent. This bivalency permits antibodies to crosslink antigens with two or more of the same epitope. Antigenic determinants that are separated by a distance can be bound by an antibody molecule. 

Diagram 4a When bivalent molecules of mAb are reacting with epitopes on a complex, the spacing of the epitopes has a profound effect on the actual affinity of the mAb. When epitopes are spaced too far to allow bivalent binding, effectively a single Fab interaction takes place. The orientation of the epitopes is also important. 

---