Final answer:
Excess antigen leading to suboptimal immune complex formation and reduced precipitation is called the prozone effect. It contrasts with the equivalence zone, where the optimal antigen-antibody ratio results in maximal precipitation.
Step-by-step explanation:
The phenomenon of excess antigen affecting the visible reaction in a neutralization assay is known as the prozone effect. In prozone, a high concentration of antigen leads to an insufficient amount of antibody to facilitate the formation of immune complexes that are large enough to precipitate, which is why the amount of precipitin decreases with excess antigen. This is in contrast to the equivalence zone, where the optimal ratio of antigen to antibody results in maximal precipitation. This is visually represented in methods like the Ouchterlony test, where a precipitin arc forms at the zone of equivalence.
In terms of understanding antibody responses, it's important to note that the most abundant antibody class detected in the serum upon reexposure to a pathogen or in reaction to a vaccine is immunoglobulin G (IgG).