Immunodiffusion in gels.

Immunodiffusion in gels encompasses a variety of techniques that are useful for the analysis of antigens and antibodies (1). The fundamental immunochemical principles behind their use are exactly the same as those that apply to antigen#x2013;antibody interactions in the liquid state. Thus an antigen will rapidly react its specific antibody to form a complex, the composition of which will depend on the nature, concentrations, and proportions of the initial reactants. As increasing amounts of a multivalent antigen are allowed to react with a fixed amount of antibody, precipitation occurs, in part because of extensive crosslinking between the reactant molecules. Initially the antibody is in excess and all of the added antigen is present in the form of an insoluble antigen#x2013;antibody aggregate. Addition of more antigen leads to the formation of more immune precipitate. However, a point is reached beyond which further addition of antigen produces an excess of antigen and leads to a reduction in the amount of the precipitate (see Fig. 1) because of the formation of soluble antigen#x2013;antibody complexes. Fig. 1. Variation in the amount of immune precipitate on addition of increasing amounts of antigen to a fixed amount of antibody.