Which antibody forms a pentamer in circulation?

Which antibody forms a pentamer in circulation?

• A. IgG
• B. IgM
• C. IgA
• D. IgE

Answer: (B)

IgM forms a pentamer in circulation. This pentamer is made of five monomer units held together by a joining (J) chain, giving ten antigen-binding sites and very high overall avidity. The large, multivalent structure makes IgM especially efficient at activating the classical complement pathway, which is crucial early in an immune response. Its size keeps it predominantly in the bloodstream rather than diffusing into tissues. In contrast, IgG is a monomer that circulates widely, IgA is usually a dimer in secretions (secretory IgA) but is a monomer in serum, and IgE is a monomer involved in allergic responses.

IgM forms a pentamer in circulation. This pentamer is made of five monomer units held together by a joining (J) chain, giving ten antigen-binding sites and very high overall avidity. The large, multivalent structure makes IgM especially efficient at activating the classical complement pathway, which is crucial early in an immune response. Its size keeps it predominantly in the bloodstream rather than diffusing into tissues. In contrast, IgG is a monomer that circulates widely, IgA is usually a dimer in secretions (secretory IgA) but is a monomer in serum, and IgE is a monomer involved in allergic responses.