Polymerization of antibodies – the J chain: In human immunology, the joining (J) chain polypeptide is associated with which immunoglobulin classes in their polymeric forms?

Introduction / Context:
The J (joining) chain is a small polypeptide critical for the assembly and secretion of polymeric antibodies. It influences mucosal immunity and complement activation by controlling antibody valency and transport. This question checks recognition of which isotypes carry J chain in their typical multimeric states.

Given Data / Assumptions:

• IgA is secreted predominantly as a dimer across mucosal surfaces.
• IgM is secreted as a pentamer in serum during early responses.
• The J chain links monomers to form stable polymers and enables transcytosis of IgA.

Concept / Approach:
Dimeric secretory IgA incorporates one J chain to connect two IgA monomers and to bind the polymeric immunoglobulin receptor for epithelial transcytosis. Pentameric IgM contains one J chain that stabilizes the pentamer and contributes to its distinctive star-shaped structure and strong avidity. IgE and IgD are predominantly monomeric and do not normally include J chain.

Step-by-Step Solution:
Identify polymeric isotypes: IgA (dimer) and IgM (pentamer). Recall that J chain is required for their assembly and transport. Select “Both IgA and IgM.”

Verification / Alternative check:
Secretory IgA includes the secretory component in addition to the J chain; IgM’s high complement-fixing efficiency reflects its pentameric architecture supported by J chain.

Why Other Options Are Wrong:
IgA only or IgM only are incomplete; IgE and IgD do not typically associate with J chain.

Common Pitfalls:
Confusing the J chain with light-chain “J segments” (gene segments) or with the secretory component of IgA; they are distinct entities.

Final Answer:
Both IgA and IgM.