Divalent binding of monoclonal antibody to a cell surface antigen. Modelling of equilibrium data.

The principles of the binding of a monoclonal Ab to a single epitope of a cell surface antigen were deduced. The equilibrium between the Ab and the antigen was regarded as a two-step reaction, involving free Ab and antigen as well as Ab, monovalently or divalently complexed to its antigen. By a few simple equations the concn of bound Ab was calculated from the values of free Ab concn, cell concn, single cell antigen density and the two equilibrium constants. This cell surface binding was compared to the binding of an ordinary one-step equilibrium reaction. It was shown that a bonus effect due to cell surface divalent binding is obtained when the product of the first association constant and the free Ab concn is less than one and, simultaneously, the product of the second equilibrium constant and the single cell antigen density is greater than one. In addition, when these conditions are fulfilled, an increase in the single cell antigen density may cause an increase in Ab binding that is much more than proportional to the change in antigen density. The implications of these results to tumor immunolocalization and immunotherapy are discussed.