A typical application of the mixed-aggregation immunocytochemical method: detection of lactic-dehydrogenase isozymes in human tissues.

To localize and quantify an enzyme in a tissue section, it is essential that all enzymes remains at its in vivo site and that the method of detection used is selective. These requirements can be fulfilled by using antibodies: (1) Bivalent IgG acts a bifunctional cross-linking reagent. (2) The IgG reacts only with some antigens, the specificity of which can be determined otherwise. (3) Nonreacting cytoplasmic antigens diffuse into the incubation medium, thereby reducing the background. (4) IgG in antigen-antibody complexes of sections will bind soluble antigen added to these sections in a second incubation step, since antigens have several antigenic sites, only some of which cross-link with neighbouring antigens. (5) Binding of soluble antigen is a function of the concentration of the complex in the tissue section and the soluble antigen, increasing sensitivity. (6) The newly introduced antigen can be visualized either by a chemical label at the antigen site or by its catalytic activity. The application of such a technique is demonstrated with the localization of lactic dehydrogenase isozymes in human tissue. It is discussed in the context of previously published results obtained by this technique, but with different enzymes.