Surface immune precipitation: a new method for rapid quantitative antigen analysis

A new method for quantitative antigen analysis is described which precludes necessity for long incubation periods. The new technique, Surface Immune Precipitation (SIP), entails the application of an antigen sample droplet directly onto the surface of a gel containing antibody, the gel being supported by a reflecting substrate. Within a few minutes, a visible uniform opaque precipitate area appears at the location of the antigen droplet when antigen-specific antibody is present in the gel. The reflecting substrate enhances the contrast between the precipitate area and the surrounding gel. The scattering intensity of the precipitate, which may be measured with simple photodetection apparatus, is related to the antigen concentration in the droplet. Good correlation is demonstrated between the Surface Immune Precipitation method and radial immunodiffusion for the analysis of serum IgG.