IgM exists as which structural form in its secreted antibody?

IgM exists as which structural form in its secreted antibody?

• A. Monomer
• B. Pentamer
• C. Trimer
• D. Dimer

Answer: (B)

IgM that is secreted is organized as a pentamer. Five IgM monomers come together and are linked by a joining (J) chain, forming a large pentameric antibody with ten antigen-binding sites. This high valency makes secreted IgM especially good at agglutinating pathogens and efficiently activating the classical complement pathway through C1q binding. In contrast, the IgM displayed on B cells as a membrane receptor is a single unit (monomer), not the secreted pentamer. (Note: hexameric IgM can occasionally occur when the J chain is absent, but the typical secreted form is pentameric.)

IgM that is secreted is organized as a pentamer. Five IgM monomers come together and are linked by a joining (J) chain, forming a large pentameric antibody with ten antigen-binding sites. This high valency makes secreted IgM especially good at agglutinating pathogens and efficiently activating the classical complement pathway through C1q binding. In contrast, the IgM displayed on B cells as a membrane receptor is a single unit (monomer), not the secreted pentamer. (Note: hexameric IgM can occasionally occur when the J chain is absent, but the typical secreted form is pentameric.)