A new method, which allows to evaluate parameters of interaction between antibodies (or receptors) and an antigen (or ligand) is suggested. The method is based on the use of so-called coordinates of dilution suggested by the author earlier. Representation of the data of the titration curves for the mixtures of antibodies (or receptors) and antigen (or ligand) in these coordinates allows one to determine the affinity of interaction and the concentration of antigen (or ligand), which can reversibly block antibodies (or receptors). Simple formulas, which allow to estimate which part of paratopes or bivalent antibodies is free and which part is blocked by the antigen, depending on dilution of the considered system, are also suggested. Such a method could be useful for characterization of infection and autoimmune processes when the antigen and antibodies circulate together in the bloodstream.