## 1 Introduction  
The possibility of obtaining antibodies that can bind very different compounds has led to the intense development of immunoanalytical systems. In modern practice, various forms of immunoassay are successfully used in medical diagnostics, quality and safety control of consumer products, environmental monitoring, etc. [1; 2; 3; 4]. The advantages of immunoassayal methods include simple sample preparation and assay implementation, highly productive testing, and the absence of the need for sophisticated equipment and highly trained operators. In many cases, immunoassays are adapted for autonomous realization directly at sampling locations, i.e., point-of-care (POC) diagnostics [5; 6; 7]. Depending on the tasks to be solved and the available equipment, various analysis formats can be realized with different labels and instrumentation. These formats of immunoassayals include enzyme immunoassay, immunoagglutination, lateral-flow and through-flow membrane immunoassayals, systems that detect the intensity or polarization of fluorescence, electrochemical immunosensors, etc. [8; 9]. Despite the ability of antibodies to detect target antigens in complex matrices with high recovery, immunoassays have significant limitations. First of all, due to the structure of an antibody's binding site and its mechanisms of antigen binding, it can potentially reveal the presence of a number of chemically similar substances rather than a single compound. Occasionally, information on the total content of all such compounds is of practical interest because of their similar biological actions. In other cases, it is necessary to identify a strictly defined substance or a minimum number of its derivatives with high functional activity against the background of other structural analogs not possessing this activity. However, the result of an immunoassay is the total number of formed and detected immune complexes, without data on which antigen variants were involved in their formation [8; 10]. Therefore, for analysis, a decision on the required specificity is made in each case based on the specific task. Reactants can be searched for either high-selective (responding strictly to a specific compound) or low-selective (responding to a number of similar compounds) immunoassays. To estimate the detection of chemically similar substances by immunotechniques, cross-reactivity (CR) is the most commonly used parameter [11]. Considering the detection of a main analyte and a cross-reactant, the ratio of their concentrations causing the same analytical signal is considered a measure of cross-reactivity. In the case of competitive immunoassay formats (which are typically used for the detection of low molecular weight monovalent antigens), a generally accepted recommendation for maximal measurement accuracy is to calculate the cross-reactivity as the ratio of the concentrations causing a 50% decrease in the detected signal [8; 12]:  
Cross-reactivity (CR) = IC50(target analyte)/IC50(tested cross-reactant) \(\times\) 100%  
The search for an assay with the necessary parameters of cross-reactivity may be realized as a simple screening of different available antibodies (comparison of reagents from different manufacturers, transition from monoclonal to polyclonal drugs or vice versa, the formation of mixtures of antibodies [13]), but when using the same immunogen, the variability of antibody selectivity is limited. Moreover, such screening requires access to a large number of preparations and is limited by financial factors when working with commercially prepared antibodies. A number of approaches have been presented in the literature to date that allow for a targeted influence on the selectivity of immunoassays. The most common is to design an antigenic preparation for the best presentation of either unique or common epitope(s) for antibody generation [14; 15; 16; 17; 18]. In a number of cases, the results of such a design have been successful and made it possible to obtain antibodies with the required spectrum of selectivity.