Question

J-polypeptide chain is structural component of –

(A). IgG
(B). IgA
(C). IgM
(D). IgD

Choose the correct answer from the options given below:

• A. Only (C)
• B. (A), (B) and (D) only.
• C. Only (B).
• D. (B), and (C) only.

Hint:

Associate the J (Joining) chain exclusively with the polymeric antibodies: dimeric IgA (found in mucosal secretions) and pentameric IgM (found in primary blood circulation).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Most immunoglobulins circulate as monomeric units. However, specific classes form polymeric structures to facilitate transport across mucosal membranes or maximize antigen binding efficiency.

Step 2: Detailed Explanation:
1. The J chain (Joining chain) is a small acidic polypeptide of approximately \(15\text{ kDa}\).
2. It plays a critical role in facilitating the polymerization of secreted immunoglobulins.
3. Specifically, it covalently binds via disulfide bridges to:
- Dimeric or polymeric IgA (B)
- Pentameric IgM (C)
It is absent from monomeric immunoglobulins like IgG (A), IgD (D), and IgE.
Therefore, the J-chain is a structural component of both IgA and IgM.

Step 3: Final Answer:
The correct option is 4, which corresponds to (B), and (C) only.