ABSTRACT
The divalency of antibody and antigen molecules can easily be expected to lead to formation of cyclic complexes, so that linear chains of various sizes are in equilibrium with the corresponding rings. Ring closure occurs by a monomolecular reaction within a single antigen-antibody complex. In the interaction of divalent antigen with divalent antibody, formation of cyclic complexes results in an increase of the overall binding efficiency which is modulated by the total concentration of antibody sites. In the Hill plot, the logarithmic scale allows a better visualization of these opposite pseudo-cooperativities. The chapter develops the study of a ternary system in which two different divalent antigens interact with hybrid antibodies. It shows that how the binding of one antigen can be enhanced by the presence of the other one thus outlining the rationale for the design of an enzyme immunoassay.
