Immunodiffusion is a serological assay in which antigen and antibody diffuse through a gel matrix and form visible precipitin lines when they meet in optimal proportions.
Principle and Types
The technique of immunodiffusion relies on the passive movement of soluble antigen and antibody molecules through a semi‑solid medium such as agar or agarose. As the molecules diffuse, they form a concentration gradient; when zones of antigen and antibody of equivalent concentration overlap, immune complexes precipitate and become visible as lines in the gel. There are two main formats. In radial or single immunodiffusion, antibody is uniformly incorporated into the gel and wells are filled with antigen samples. Antigen diffuses outward in all directions, and the diameter of the precipitin ring is proportional to the antigen concentration, allowing quantification of proteins such as immunoglobulins or complement components. In double diffusion, developed by Ouchterlony, both antigen and antibody are loaded into separate wells and diffuse toward each other. The pattern of precipitation lines reveals whether antigens are identical, partially related or different, providing qualitative information about antigenic relationships. Variants such as immunoelectrophoresis combine electrophoretic separation with diffusion to increase resolution. Although immunodiffusion assays are simple and inexpensive, they are less sensitive than methods using labeled antibodies and require several hours or days for clear bands to develop.
Laboratory Uses and Interpretation
Radial immunodiffusion is used in clinical laboratories to measure concentrations of serum proteins, including IgG, IgA and complement components, by comparing ring diameters to a standard curve. Double immunodiffusion assays help identify fungal pathogens, detect autoantibodies and compare vaccine antigens. In an Ouchterlony test, a pattern of continuous lines between antigen wells and an antibody well indicates identity, while lines that cross or form spurs denote non‑identity or partial identity. These assays require careful preparation of the gel and proper incubation to obtain interpretable results. Despite being largely supplanted by more rapid immunoassays, immunodiffusion remains valuable for teaching immunology and for applications where cost or equipment constraints preclude other methods. Immunodiffusion exploits the natural tendency of antigens and antibodies to form precipitates when present at appropriate ratios. By observing the formation and pattern of precipitin lines, researchers and clinicians can quantify proteins and assess antigenic relationships using straightforward equipment. Related Terms: Precipitin, Immunoelectrophoresis, Radial immunodiffusion, Ouchterlony assay, Agarose