The Immunoglobulin J chain (IGJ) is a critical 137-amino acid polypeptide essential for antibody polymerization and mucosal immunity. Located on chromosome 4, it links multiple immunoglobulin molecules, specifically connecting two IgA molecules or five IgM molecules through disulfide bonds.

Its primary functions include:

• Facilitating immunoglobulin polymerization
• Enabling transcytosis of antibodies across mucosal surfaces
• Enhancing antibody binding strength and complement activation

In mucosal immunity, the J chain is crucial for:

• Producing secretory IgA (sIgA)
• Protecting against pathogens in digestive and respiratory tracts
• Neutralizing viruses and toxins
• Maintaining microbiome balance

The J chain's significance extends to:

• Interaction with polymeric immunoglobulin receptor (plgR)
• Transporting antibodies across mucosal barriers
• Providing immune protection in breast milk for infants

Clinical implications include:

• Potential role in IgA deficiencies
• Emerging therapeutic applications
• Biotechnological approaches to modify antibody delivery

Absence of the J chain significantly impairs antibody transport and mucosal defense, highlighting its critical role in immune system function.