Stratification and compartmentalisation of immunoglobulin responses to commensal intestinal microbes

• Bacteria are stratified transversely and longitudinally within the intestinal tract.
• IgA influences mutualism between the host and its normal microbiota.
• IgA can be shaped to the presence of the dominant microbial species.
• Mucosal and systemic Ig responses against commensal bacteria are compartmentalized.

The gastrointestinal tract is heavily colonized with commensal microbes with the concentration of bacteria increasing longitudinally down the length of the intestine. Bacteria are also spatially distributed transversely from the epithelial surface to the intestinal lumen with the inner mucus layer normally void of bacteria. Maintenance of this equilibrium is extremely important for human health and, as the dominant immunoglobulin at mucosal sites, IgA influences mutualism between the host and its normal microbiota. In this review we focus on the links between immune and microbial geography of the mammalian intestinal tract.