Development

The discovery that antigen-antibody interaction could be produced not only in liquids, but also on gel media, such as agar or agarose gels, with the formation of insoluble immunoprecipitates, opened the door to the development of the gel diffusion techniques for immunoprecipitation analysis. The first of these techniques, known as double diffusion, was introduced by Ouchterlony in 1948. In this technique, the antigens (proteins) and the corresponding antibodies (immunoglobulins, Ig) are located on a thin agar gel, in small and separated wells. The simple diffusion of the antigen and the antibody produce precipitation lines between the two wells where the interaction of these molecules occurs.

Advances in gel immunoprecipitation techniques occurred in 1953, when Grabar and Williams described immunoelectrophoresis (IE), in which the high resolution of electrophoresis and the specificity and sensitivity of the immunological reactants are combined. The immunoelectrophoretic techniques had a very rapid development during the 1960s and early 1970s, because they are adequately suited to the analysis of complex mixtures of proteins. The first attempts to improve the immunoelectrophoretic technique pursued two main objectives: to increase its speed (IE was rather slow, mainly due to the time-consuming immunodiffusion step) and to achieve quantitative methods (IE is basically qualitative).

In a short period of time new techniques based on crossed immunoelectrophoresis (CIE), rocket immunoelectrophoresis (RIE), counter IE, crossed-affinity IE (CAIE) and charge shift IE, were introduced.

This article describes all the techniques in which electrophoresis and immunoprecipitation steps performed in agar or agarose gels are combined. Diffusion-type techniques, such as double immunodiffusion and the quantitative radial immunodiffusion, will not be described. Other immunochemical techniques, such as enzyme-linked immunosorbent assay (ELISA) and Western blotting, in which immunoprecipitation is not produced, will be considered elsewhere.

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