Final answer:
The Ouchterlony assay and radial immunodiffusion assay are methods for detecting antibody-antigen interactions, with the former for visualization and the latter for quantification of antigens.
Step-by-step explanation:
The Ouchterlony assay and radial immunodiffusion (RID) assay are both techniques used to detect antibody-antigen interactions, commonly referred to as precipitin reactions. In the precipitin ring test, visible rings form when soluble antigens and antibodies create large complexes or lattices. However, rings may not form if there's a lack of one of the reactants or improper conditions. The Ouchterlony assay, developed by Orjan Ouchterlony in 1948, involves placing antigens and antibodies in neighboring wells in an agar gel to visualize precipitin arcs. In contrast, the RID assay is a quantitative method where antigens diffuse in an agar gel containing antibodies to form a precipitation zone, with the area of the zone being proportional to antigen concentration.