Final answer:
In antigen-antibody reactions, a titration is used to determine the serum antibody quantity using tests like the precipitin ring test and agglutination assays, where a visible precipitin ring or clumping indicates successful reactions, and titer is calculated based on the highest successful dilution.
Step-by-step explanation:
When antigen-antibody reactions are used to read a titration, they determine the quantity of antibodies in the serum. The process typically involves the use of a precipitin ring test or agglutination assays to measure the concentration of antibodies. In a precipitin ring test, a precipitin ring forms at the interface between a solution containing a standard antigen and a serial dilution of antiserum. The presence of a precipitin ring indicates a successful antigen-antibody reaction, and the highest dilution that still shows the ring determines the antibodies' titer. In agglutination, visible clumping occurs due to antibody-mediated cross-linking of antigens on cells or particles. Like with precipitin tests, the titer can be found by serial dilution.