20] Single radial immunodiffusion

Abstract

This chapter describes the different aspects of single radial immunodiffusion (SRID). SRID is an immunodiffusion technique, in which a single partner of the antigen–antibody reaction, usually the antigen (Ag), diffuses radially from a small well punched into a gel layer of constant thickness and the other partner, usually the antibody (Ab), has been uniformly incorporated into this gel layer and is, therefore, not considered as diffusing. The procedure of filling the Ag wells to the brim without measuring the volume is not recommended as it introduces comparatively large volumetric inaccuracies from well to well. The measurements of sharp precipitin rings may already be possible on the wet gel plates, either by using an ocular with a microcomparator scale or by enlarging the plate in a photographic or other optical enlarger. Some antigens may display the molecular size heterogeneity, usually consisting of a series of polymers, giving the reactions of immunological identity with the monomers. SRID may be used in a reversed version— that is, with diluted Ag incorporated into the gel layer and solutions of Ab or antiserum, diffusing from the wells.