In a double immunodiffusion (Ouchterlony) test, what is the ‘zone of equivalence’?

• A. A zone created by low ionic strength in the gel that prevents diffusion, causing antibody to remain concentrated in the well.
• B. The area near the antigen well where very high antigen concentrations produce a diffuse cloud of precipitate rather than a discrete line.
• C. The central point of the plate where diffusion stops and no interaction can occur because antibody has been completely consumed.
• D. The region in the gel where antigen and antibody concentrations are optimal and balanced such that a visible precipitin line forms due to lattice formation.

Answer: (D)