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.

If a patient's RBCs agglutinate with both anti‑A and anti‑B sera, which blood type is indicated?

A clinician wants to know if a patient is actively shedding a respiratory virus right now. Which FA format is appropriate and why?

Design an FA-based workflow to determine if a patient has a current bacterial infection versus only past exposure. Which of the following experimental designs would best distinguish the two conditions?

You test two patient sera in parallel with twofold serial dilutions starting at 1:20. Serum A is positive through 1:640 but negative at 1:1280. Serum B is positive through 1:160 but negative at 1:320. What is the fold difference in antibody titer between Serum A and Serum B and what does this imply?