Question

Short answer:
The Fc receptor FcRn,
expressed in intestinal epithelial cells,
binds to the Fc portion of IgG antibodies.
Moreover, FcRn binds to IgG with high
affinity at an acidic pH (<6.5) but not at
physiological pH (pH 7.4). How is this
biochemical feature of FcRn critical for its
role in antigen presentations for gut
immunity?

Answer 1

The binding of IgG to FcRn occurs in acidic environments and release occurs at neutral pH in the tissues and plasma. The interaction is pH dependent, with low pH favoring association and neutral pH favoring dissociation.

• FcRn is comprised of regions ?2 domain and ?2-m that binds to the region between the C?2 and C?3 domains of IgG Fc.
• The FcRn–IgG Fc interaction is stabilized by salt bridges involving histidine residues in the Fc and aspartate and glutamine residues on FcRn.
• Histidine protonation at low pH results in salt bridge formation, while at pH values above 7 the histidines are neutral and the bridges dissolve.

FcRn are present in Anigen Presenting Cells (APCs), is responsible for -

• Luminal antigens binds to IgG forming IgG-antigens complex and deliver these to local dendritic cells, that functions as APC, which inturn stimulates the activation of adaptive immune response, CD4+ and CD8+ T cell responses.
• Transport of ingested IgG from the intestinal lumen into the circulation,
• Transport of maternal IgG to the fetus,
• Transfer of IgG out into mucosal secretions, and
• Regulation of IgG