Figure 1.

Illustration of intestinal barrier structure and functions. The intestine barrier contains the chemical barrier and the physical barrier. The chemical barrier is composed of antimicrobial peptides (AMPs) such as amphiregulin. It provides chemical agents attacking invading microorganisms including bacteria and helminths. The physical barrier includes the mucus layer and cell junctions between the epithelium. It serves as the wall spatially separating the invading microorganisms and host. There are many types of cells in the gut epithelium regulating the epithelium function. Disruption of the intestinal barrier allows the leak of gut bacteria from the lumen into the lamina propria, inducing excessive immune responses of the host immune cells. Retinoic acid (RA) released by macrophages or dendritic cells assists in host resist helminthic infection. IL-22 released by ILCs promotes epithelial cells secreting AMP in response to bacterial infection, which is regulated by IL-23 from dendritic cells. Moreover, macrophage-derived IL-1β promotes ILCs' production of GM-CSF, which further stimulates more macrophage differentiation from monocytes. The enteric nervous system including neuron and glial cells interacts closely with mast cells and regulates blood vessels. IL: interleukin; AMP: antimicrobial peptide; GM-CSF: granulocyte-macrophage colony stimulating factor; RA: retinoic acid; ENS: enteric nervous system; CNS: central nervous system.