Final answer:
IgG is the predominant antibody during the late primary and secondary immune responses, capable of clearing pathogens, activating complement, and crossing the placenta to provide immunity to the developing fetus. It is secreted by B-lymphocytes after antigen recognition and class switching.
Step-by-step explanation:
IgG is the main antibody found in the blood during the late primary immune response and continues to be the predominant antibody in secondary responses. This is attributed to the process of class switching that occurs after the initial response. IgG operates as a monomeric antibody, which not only helps clear pathogens from the blood but also has the ability to activate complement proteins. Although IgG triggers complement activation less effectively compared to IgM, it is still a significant component of the body's antibacterial defences.
In addition to its role against pathogens in the blood, IgG is of particular importance because it can cross the placenta, thereby granting passive immunity to the developing fetus. Moreover, IgG can leave the bloodstream and enter interstitial fluid to combat extracellular pathogens. B-lymphocytes, through their cell surface receptors, recognize antigens and secrete IgG into the bloodstream to perform its various protective functions.