Immunodiffusion

Immunodiffusion reactions are diagnostic tests that use the diffusion of particles through a medium (such as agar) to measure:

• Relative concentrations of antibodies/antigens
• Relative purity of an antigen preparation

There are two types of immunodiffusion reaction, both being carried out on a semisolid medium:

• Radial immunodiffusion: also known as the Mancini method. The medium is prepared with a suitable dilution of antiserum and the antigen sample placed on top and allowed to diffuse. As diffusion takes place, large insoluble complexes form creating a ring of precipitation (precipitin). By comparing the area of the ring with a standard curve, the concentration of the antigen sample can be determined.
• Double immunodiffusion: also known as the Ouchterlony method. Antigen and antibody preparations are placed on the medium and both allowed to diffuse radially from the wells towards each other. This establishes a concentration gradient, forming an area of equivalence and line of precipitin.