The major class of immunoglobulin found in adult human serum is

• A. IgA
• B. IgE
• C. IgG
• D. IgM

Answer: (C)

IgG is the dominant immunoglobulin in adult serum because B cells switch to this isotype after initial exposure and it remains in circulation at high levels due to its long half-life and stability. As a monomer, IgG can diffuse readily into tissues and carries out key systemic defense roles, including neutralizing toxins and viruses, opsonizing pathogens for phagocytosis, and activating the classical complement pathway. It also uniquely crosses the placenta, providing passive immunity to the fetus. In contrast, IgM is mainly produced early in a primary response and exists as a pentamer, giving high avidity but limited tissue diffusion; IgA is primarily found in mucosal secretions as secretory IgA; and IgE exists in much lower concentrations and is involved in allergic responses and defense against parasites. Therefore, IgG best fits as the major class found in adult human serum.