- The correct answer is IgG because it is the most abundant immunoglobulin in human blood serum, constituting about 75-80% of the total antibody pool.

- IgG plays a crucial role in long-term immunity and pathogen neutralization due to its high concentration and ability to cross the placenta, providing passive immunity to the fetus.
- Other immunoglobulins like IgA, IgE, and IgM are present in lower concentrations or primarily localized to other body fluids.

Reference: Janeway's Immunobiology, 9th Edition, Chapter 4