Antibodies can combine with soluble antigens to form a visible precipitate. This phenomenon is the basis for a number of diagnostic tests. The precipitation reaction takes place in two stages. First, within seconds after they are mixed, antigen and antibody molecules collide and form small primary immune complexes. An immune complex is, by definition, a combination of antigen and antibody, sometimes with complement included. Second, over a period lasting from minutes to hours, latticelike cross-linking between the small primary complexes causes large precipitating immune complexes to form. The mechanism of precipitation reactions is the extensive cross-linking of soluble molecules to produce a visible insoluble product. Of course, only antigens with two or more, usually multiple, epitopes can participate in the cross-linking necessary to give precipitation.
17.4 Precipitation Reactions
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