The gut acts as an important antigen-sensing organ.
Bacterial antigen is sampled and processed by the M cells, ultimately stimulating the release and maturation of a population of pluripotential stem cells or naïve CD4 helper T lymphocytes.
These cells migrate out from the lamina propria of the gut, through the mesenteric lymph nodes and thoracic duct, and into the systemic circulation as a mature line of B- and T-cell lymphocytes.
A proportion of these cells generated in the maturation of the pluripotential stem cells migrate out as mucosal-associated lymphoid tissue (MALT) to distant sites such as the lungs, genitourinary tract, breast, and lacrimal glands.
Those that return to the Peyer patches of the enteric mucosa are known as gut-associated lymphoid tissue (GALT).27-29