In Immunoelectrophoresis (IEP), what does each precipitin line indicate?

• A. A distinct antigen-antibody reaction
• B. The pH value
• C. The density of antigens
• D. The migration distance of a protein

Answer: (A)

In immunoelectrophoresis, precipitin lines form where the antibody encounters its specific antigen after the proteins have been separated by electrophoresis. Each line indicates a distinct antigen–antibody reaction—the visible precipitate marks the location where a particular antigen in the sample binds to its corresponding antibody. This is what the test is detecting: the identity of an antigen by showing a unique reaction with the antibody.

The line is not a measure of pH, nor does it directly reflect the density of antigens, and while its position relates to the antigen’s electrophoretic mobility, the defining feature of the line is the specific antigen–antibody interaction that produced it.