Final answer:
The IgG antibodies are the main subclass associated with the secondary antibody response, clearing pathogens and providing protection across the placenta.
Step-by-step explanation:
The antibodies predominantly associated with the secondary antibody response are of the IgG subclass. After the primary exposure to an antigen, immune cells undergo a process known as class switching which, during later exposures, results in a higher production of IgG antibodies. These antibodies not only have the ability to clear pathogens from the blood and activate complement proteins but also play a crucial role in protecting the developing fetus by crossing the placenta and combating extracellular pathogens in the interstitial fluid.