This technique is used to determine the concentration of an antigen, in this case horse IgG. This assay (like Ouchterlony) is also carried out in a thin layer of agarose, but here an antibody specific for horse IgG is mixed into the agarose when the plate is prepared, and samples of horse serum are then placed into each of the small visible wells. As the IgG diffuses outward it forms a precipitate with the antibody present throughout the agarose; the area of the circle of precipitate can be measured and is directly related to the concentration of antigen (IgG) in the sample.
The top row of wells contains a set of standards of increasing concentration of horse IgG (from left to right), the other wells are test samples which can be compared with the standards to determine their concentration of IgG.