The binding molecules used in almost all immunochemical methods are Ab; only in some cases other molecules are used (e.g., receptors). The fundamental characteristics of an Ab are affinity (measured in chemical–physical terms by the equilibrium constant K of the reaction in which it binds the Ag) and specificity (ability to recognize the analyte, distinguishing it from molecules with a similar structure). The Abs most used for developing immunochemical methods are immunoglobulin G (IgG).

Ab used in immunochemical determinations can be either polyclonal or monoclonal. The production of polyclonal Ab can occur by immunization of an animal with an Ag from a different animal species. In this case, the Abs, produced by different clones of plasma cells, are heterogeneous and are directed against different Ag epitopes; their affinity for the Ag presents a high variability, but it can recognize different molecular forms of the same Ag. The characteristics of polyclonal produced Ab are specific for the response obtained in the individual animal, so the production of Ab with specificity is related, over time, to the survival of the animal itself.

Monoclonal Abs are produced by a single line of plasma cells. This phenomenon is obtained in vivo in certain myelomas in which tumor degeneration of a plasma cell clone leads to the hyperproduction of monoclonal immunoglobulin. The creation of hybrids between secretory B lymphocytes capable of producing specific Ab and plasmacytoma cells, with immortal characteristics, has made it possible to produce monoclonal Ab in adequate quantities to produce diagnostic reagents, reproducible over time (Table 1).

Therefore, the specificity of the immunochemical measurement is strictly dependent on the Ab used as a binding reagent and is defined as the ability of the Ab to recognize the Ag to be determined. The methods that use polyclonal Ab, show a lower specificity than those using monoclonal Ab, which constitutes a homogeneous population of Ab directed toward a single specific epitope. The degree of specificity can be expressed as a percentage of cross reaction relative to the ability of an Ab, typical for a given Ag, to recognize another Ag, called interferer.

Table1. Characteristics of monoclonal antibodies