Radial Immunodiffusion Assay (RID) is a specialized form of diffusion in agar in which immunoglobulin that recognizes specific antibody classes or subclasses is mixed with the molten agar before it is poured into the assay plate. Wells cut into the solidified agar are then filled with a solution (purple) that contains the antibody class or subclass that will be recognized by the immunoglobulin in the agar. On the far left, wells are filled with known quantities of the matching antibody class (650 mg/dl and 1750 mg/dl).

After an overnight incubation, a hazy zone can be seen around the wells. The diameter of each zone is measured and this is plotted as diameter squared on the y-axis versus antigen concentration on the x-axis (the plot is shown on the right, above). The standard curve for the RID assay is generated using the values for the known standards. Matching the zone squared from the unknown sample with its concentration taken from the standard curve gives the antibody quantity for the unknown sample. This is an example of a relatively simple, low cost assay that is still in use because it works and requires no special equipment or radioactivity.

Biomedical Laboratory Animations Last modified 11-15-03.

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