A theoretical and experimental analysis of double diffusion precipitin reactions in gels, and its application to characterization of antigens.

The distribution of antigen and antibody in radial double diffusion systems was studied by means of materials labelled with (131)I or (14)C. These studies showed that, once a precipitate begins to form, the assumption that antigen and antibody obey the laws of free diffusion is invalid. They also showed that, in the systems used, no antigen and very little antibody diffused past the zone of visible precipitation. It was found that accurate estimates of the diffusion constants of antigens could be obtained by allowing antigen and antibody to diffuse from troughs set at right angles and by measuring the angle of the precipitin line. Examples of the use of this method, and a theoretical treatment are given. An alternative method for estimating the size of antigens is to use the `molecular sieve' properties of gelatin gels, which are sharply graded with the concentration of the gel.