A quantity determined by observing the highest dilution of serum that produces a visible reaction with an antigen is called .

The quantity you are referring to is known as the end-point titer or serum titer. In immunology, this term describes the highest dilution of serum that still produces a visible reaction when mixed with a specific antigen. This reaction is often measured in a laboratory setting, where serial dilutions of serum are prepared and mixed with a constant amount of antigen.

The importance of determining the end-point titer lies in its ability to quantify the concentration of antibodies present in the serum. A higher titer indicates a greater presence of antibodies against the antigen, suggesting a stronger immune response or past exposure to an infection or vaccination. This concept is crucial in various applications such as vaccine efficacy studies and diagnosing infections or autoimmune diseases. Overall, assessing the end-point titer aids in understanding the immune status of an individual and helps clinicians make informed decisions regarding treatment and management.