Which antibody isotype is most abundant in serum?

• A. IgA
• B. IgE
• C. IgM
• D. IgG

Answer: (D)

IgG is the most abundant antibody isotype in serum. Its high concentration reflects its role as the main systemic defender and its long persistence in circulation. IgG is produced throughout the immune response, especially during ongoing or secondary exposures, and it benefits from the FcRn receptor that rescues it from degradation, giving it a long half-life of about 21 days. This combination of steady production and extended half-life keeps IgG levels high, making it the predominant circulating immunoglobulin.

In contrast, IgM is produced early during infections and exists as a pentamer, which makes it large and largely confined to the intravascular space, so its serum levels are lower than IgG. IgA is more abundant in mucosal secretions, where it protects surfaces like the gut and respiratory tract, and is present in serum at lower levels. IgE exists in very small amounts in serum, largely bound to receptors on mast cells and basophils and involved in allergic responses and defense against parasites.