In double radial diffusion, which observation indicates that the antigens are identical?

In double radial diffusion, also known as Ouchterlony double diffusion, the detection of identical antigens is indicated by a line of fusion between the antigen and antibody wells. When two identical antigens diffuse into the gel from their respective wells, they meet and form a continuous precipitin line. This line demonstrates that the antigens are the same because the identical antigen-antibody interactions will result in a precise zone of precipitation.

The line of fusion signifies an equivalence zone where the antigen and antibody concentrations are optimal for complex formation, leading to a visual representation of the reaction. If the antigens were different, distinct lines would form, showing zones of precipitation that do not merge.

This interpretation is central to assessing antigen identity in laboratory practices involving immunodiffusion techniques. Thus, the presence of a line of fusion is a straightforward visual cue indicating that the antigens are indeed identical.