Simplified model: in a double diffusion gel of unit distance between wells, antibody diffuses from the left well with an initial concentration of 100 arbitrary units and decreases linearly to 0 at the midpoint between wells, while antigen diffuses from the right well with an initial concentration of 25 units and decreases linearly to 0 at the midpoint. If we assume linear profiles across the whole distance (left to right normalized 0 to 1), at what normalized position x measured from the left well will antibody and antigen concentrations be equal (i.e., where a precipitin line might form)? Show your calculation and final answer.

• A. At the midpoint x = 0.5 since diffusion is symmetric and precipitin lines always form halfway between wells under linear diffusion assumptions.
• B. At x = 0.8 (80% from the left well), because solving 100(1 - x) = 25x gives x = 0.8.
• C. At x = 0.2 (20% from the left well), because the lower antigen concentration pushes the line toward the antibody well.
• D. No line will form because antibody concentration is too high overall; the high antibody zone prevents lattice formation throughout the gel.

Answer: (B)