Explained in “Keeping Bacteria at a Distance” here.
An antibacterial peptide is essential for restricting contact between the intestinal microbiota and host.
Summary: The human intestine harbors enormous amounts of bacteria that have an essential role in host metabolism, but how this mutualistic balance is maintained is unclear. The current understanding has focused on the concept that bacteria continuously interact with the intestinal immune system in a balanced proinflammatory and tolerogenic way. The discovery of a protective inner mucus layer in the colon that separates bacteria from the epithelium has broadened this view (1). On page 255 of this issue, Vaishnava et al. (2) show that the antibacterial protein RegIII? secreted by specialized epithelial cells is involved in limiting the epithelial contact with bacteria in the small intestine. This observation further substantiates the role of intestinal epithelial cells and the mucus that covers them as important parts of the innate immune defense.
Someday it’s a design inference. Thoughts?