Final answer:
IgG is the most prevalent antibody in the blood upon reexposure to a pathogen or after vaccination, providing passive immunity and fighting against extracellular pathogens.
Step-by-step explanation:
The most abundant antibody class detected in the serum upon reexposure to a pathogen or in reaction to a vaccine is IgG. During a primary response, class switching allows IgG to become the major antibody. As a monomeric antibody, IgG is crucial for clearing pathogens from the blood and can partially activate complement proteins. Its exceptional ability to cross the placenta means that it provides passive immunity not just in the event of reexposure but also to the developing fetus. Additionally, IgG is able to move out of the blood and into the interstitial fluid, combating extracellular pathogens.