Which immunoglobulin is the most abundant in human serum?

• A. IgA
• B. IgG
• C. IgM
• D. IgE

Answer: (B)

IgG is the most abundant immunoglobulin in human serum because it becomes the dominant antibody produced after an initial immune response and remains in circulation the longest. After B cells switch from producing IgM to IgG, the body maintains high levels of IgG thanks to its long half-life and efficient recycling by the neonatal Fc receptor, which protects it from degradation. Its small, monomeric form also allows it to diffuse readily from blood into tissues, sustaining widespread systemic protection.

In contrast, other classes serve important but different roles and are present in lower serum concentrations. IgM is produced first in a primary response and exists mainly as a pentamer, which is excellent for rapid agglutination but limits how high its serum concentration can become. IgA is primarily found in mucosal secretions, where it protects surfaces like the gut and airways, rather than in serum. IgE is present only in trace amounts but drives allergic responses and anti-parasitic defenses.