FIG. 1. Diagnostic agar gel immunodiffusion test. Viral antigen is placed in the center well, and serum samples are placed in the surrounding wells. Note, b and d are negative; all other samples are positive.
During an immune response to viral infection, the host often produces antibodies that bind to viral antigens and may inhibit the virus from initiating infection of host cells. There are many diagnostic tests that have been developed to assay host serum for the presence of antibodies, including enzyme-linked immunosorbent assays, complement fixation tests, hemagglutination inhibition tests, neutralization assays, and the agar gel immunodiffusion test.
The double agar gel immunodiffusion test (Ouchterlony method) can be used to diagnose viral infections. Viral (or other microbial) antigen is placed in the center well, and serum samples are placed in the wells surrounding the center well. The reactants will diffuse through the agar at a rate inversely related to their molecular weights. If antibodies to the antigen are present, a precipitate will be seen between the center well and sample well; the antibodies in the serum diffuse out and bind antigen that is diffusing from the center well to form large insoluble complexes. In the figure shown, all samples are positive except b and d.
In the double agar gel immunodiffusion test, both antibody and antigen are diffusing. This test only indicates that an antibody has been produced to a given virus. However, it does not prove that antibody was produced recently. It provides a cheap, quick way to determine if an infected individual has had contact with a virus thought to be causing an outbreak (1). Due to the inability of agar gel immunodiffusion to quantify antibody titers, it is no longer used as a diagnostic test in human disease but is still used for veterinary purposes.