**Core Concept:** Pentameric antibodies are immunoglobulins (Ig) composed of five polypeptide chains, including two heavy chains (μ or IgG) and three light chains (λ or κ). J chain is a 6KDa protein that plays a crucial role in the assembly, stability, and transport of immunoglobulins, particularly IgA and IgD, which lack the constant region domains needed for direct interaction with the heavy chains.

**Why the Correct Answer is Right:** The correct answer is IgA (alpha-chain immunoglobulin), as IgA is the only immunoglobulin that primarily contains the J chain. IgA is a pentameric antibody and consists of two heavy chains (μ) and three light chains (λ). J chain is essential for its assembly, stability, and transport across the mucosal surfaces.

**Why Each Wrong Option is Incorrect:**

A. IgM (m-chain immunoglobulin) does not contain J chain, making it an incorrect choice. IgM is a pentameric antibody with two heavy chains (μ) and three light chains (λ or κ).
B. IgD (delta-chain immunoglobulin) does not contain J chain, making it another incorrect choice. IgD is a pentameric antibody with two heavy chains (μ) and one light chain (λ).
C. IgE (epsilon-chain immunoglobulin) does not contain J chain, making it an incorrect choice. IgE is a pentameric antibody with two heavy chains (μ) and one light chain (κ).
D. IgA lacks the J chain, which is essential for its assembly and transport, making it incorrect. IgA contains the J chain, as explained above.

**Clinical Pearl:** The presence of J chain in IgA is crucial for its function in mucosal immunity, particularly in the respiratory and gastrointestinal tracts. This highlights the role of J chain in facilitating the assembly and transport of IgA, ensuring its efficient function at mucosal surfaces.