Which immunoglobulin is the largest in size and forms pentamers?

• A. IgA
• B. IgG
• C. IgM
• D. IgE

Answer: (C)

Immunoglobulins differ in size and structure, and the one that sits at the largest size and is organized as a pentamer is IgM. It is produced early in the immune response and circulates mainly as a pentamer formed by five basic antibody units connected by a J chain. This pentameric arrangement gives IgM ten antigen-binding sites, greatly enhancing its ability to bind and agglutinate antigens and to activate the classical complement pathway efficiently. Because of its large size, IgM stays in the bloodstream and does not cross the placenta, unlike the smaller IgG. In contrast, IgA is typically a dimer in secretions, IgG is a monomer that crosses the placenta, and IgE is a monomer involved in allergic responses. So the feature of being the largest and forming a pentamer identifies IgM.