If crossed lines result in an Ouchterlony immunodiffusion reaction with antigens 1 and 2, what does this indicate?

• A. Antigens 1 and 2 are identical.
• B. Antigen 2 is simpler than antigen 1.
• C. Antigen 2 is more complex than antigen 1.
• D. The two antigens are unrelated.

Answer: (D)

In Ouchterlony immunodiffusion, the pattern of precipitation lines reveals how related the antigens are to the antibody. When two diffusion lines come from different antigen wells and cross instead of fusing into a single line, it means there is no shared epitope recognized by the antibody between the two antigens—their antigenic determinants aren’t the same, so they don’t form a common precipitin line. This is why crossed lines indicate that the two antigens are unrelated. If they were identical, the lines would merge into one; if they shared some epitopes but were not identical, you’d see a spur indicating partial identity.