IgM contains which structural component in its polymer form?

• A. Secretory component
• B. Hinge region
• C. J chain
• D. Carbohydrate side chain

Answer: (C)

Polymeric IgM is assembled as a pentamer, with five IgM units joined by a single J chain. This small polypeptide acts as the connector that links the Fc regions of adjacent IgM molecules, creating the polymeric form. The J chain is essential for polymerization and also helps recruit the polymeric immunoglobulin receptor (pIgR) to transport the molecule across epithelium into secretions; once transcytosed, a portion of the receptor—known as the secretory component—remains bound to the complex in secretions. So, the component that defines the polymeric form of IgM is the J chain. The secretory component is involved in secretion after transport, not in holding the IgM monomers together; the hinge region and carbohydrate side chains are not what establish the polymeric structure.