What is the principle of Immunoelectrophoresis (IEP)?

• A. A change in pH separates enzymes
• B. An electric current is applied to a gel, antigens separate by charge and size; precipitin lines indicate antigen–antibody reactions
• C. Magnetic fields separate proteins by charge
• D. Centrifugation separates antigens by density

Answer: (B)

Immunoelectrophoresis combines two ideas: separating proteins in a gel by an electric field and then visualizing antigen–antibody interactions as visible lines. When the gel has an electric current, proteins migrate according to their charge and size, creating distinct bands. Then antibodies are allowed to diffuse into the gel and bind to their matching antigens, forming insoluble immune complexes that appear as precipitin lines. These lines indicate where specific antigen–antibody reactions have occurred, providing a profile of the immune components in the sample. The other options describe different separation methods—changing pH to separate enzymes, magnetic field separation, or density-based centrifugation—that are not how immunoelectrophoresis works.