Final answer:
Approximately 75-80% of circulating antibodies in serum are IgG, which is the most abundant antibody in the human blood and plays a key role in immune defense.
Step-by-step explanation:
Approximately 75-80% of circulating antibodies in serum are IgG. This class of antibody is predominant due to its versatility and abundance in the human blood. IgG antibodies are notable for their ability to cross the placental barrier, providing passive immunity to the developing fetus, and they are crucial for the body's defense against pathogens. IgA constitutes about 10-15% of the total immunoglobulins and plays a significant role in mucosal immunity, while IgM, IgD, and IgE are less abundant in the serum.