Which of the following is a characteristic of IgG?

The characteristic of IgG being the most abundant antibody in serum is a central aspect of its role in the immune system. IgG accounts for approximately 70-75% of the total immunoglobulin pool in human serum. This predominance allows IgG to play a crucial role in long-term immunity and protection against pathogens.

Its high concentration in serum facilitates effective opsonization, neutralization of toxins and viruses, and activation of the complement system, which enhances phagocytosis and the clearance of pathogens. Additionally, IgG has various subclasses that contribute to its ability to mediate diverse immune responses.

Other choices do not correctly describe IgG: it is not the first antibody produced (that role is attributed to IgM), does cross the placenta to provide passive immunity to the fetus, and does not form a pentamer, as it is monomeric when secreted.