antibody is contained in the precipitate. This is called the antibody-excess zone (i.e. prozone phenomenon). As more antigen is added, the amount of protein precipitated increases until the antigen/antibody molecules are at an optimal ratio. This is known as the zone of equivalence or equivalence point. When the amount of antigen in solution exceeds the amount of antibody, the amount of precipitation will decrease. This is known as the antigen excess zone. References at Google Books External links Simulator at University of California, Irvine Photograph of Ouchterlony double immunodiffusion plate with unstained precipitin lines of full identity and non-identity. Photographs of Ouchterlony immunodiffusion patterns showing stained precipitin lines of full identity, partial identity and non-identity. Biological techniques and tools Diagnostic virology Immunologic tests