Which immunoglobulin class is most commonly described as having ten antigen-binding sites in its most common form?

• A. IgA
• B. IgD
• C. IgG
• D. IgM

Answer: (D)

Antibody structure and binding capacity explain why ten antigen-binding sites are associated with IgM. Each IgM unit has two Fab regions, and IgM is secreted as a pentamer joined by a J chain. This arrangement yields ten Fab arms in one molecule, giving IgM high overall binding strength (avidity) for multivalent antigens and making it especially effective at initiating the classical complement pathway early in an immune response.

In contrast, IgG and IgD exist as single units with two Fab regions, while IgA is typically a secreted dimer with four Fab arms. Therefore, IgM is the class described as having ten antigen-binding sites in its most common form.