IgA contains the J chain, a small polypeptide essential for polymerizing dimeric IgA and pentameric IgM, enabling their transport across mucosal epithelia.

Question Breakdown

This question tests immunoglobulin structure, focusing on polymeric antibodies—a core immunology topic for life sciences students preparing for GATE exams, linking antibody function to mucosal immunity.

Option Explanations

• IgE: Monomeric; binds mast cells/basophils via FcεRI for type I hypersensitivity (allergy/parasite defense); lacks J chain as it doesn’t polymerize.

• IgA: Correct; secretory/dimeric IgA (sIgA) incorporates one J chain per dimer, facilitating pIgR binding for transcytosis into mucosa (e.g., saliva, tears, milk).

• IgG: Monomeric (~150 kDa); primary serum antibody for opsonization/complement activation; no J chain required for its function.

• IgD: Monomeric surface receptor on naive B cells; co-expressed with IgM; no polymeric form or J chain involvement.

Introduction to J Chain Location

J chain is present in polymeric IgA and IgM, regulating their multimerization and secretory transport via polymeric immunoglobulin receptor (pIgR). Absent in monomeric IgG/IgE/IgD.

J Chain Structure and Role

15-16 kDa polypeptide (137 aa, 8 cysteines); forms disulfide bonds with heavy chain tailpieces (Cα4 in IgA, Cμ4 in IgM). Creates pIgR binding site for epithelial transcytosis; ERp44 ensures assembly quality control.

Immunoglobulin Comparison

J chain defines secretory competence.

Applications in Microbiology

Critical for vaccine design (mucosal IgA), B cell differentiation studies; IgM pentamers dominate primary responses with J chain-stabilized avidity.