Page 670 - Veterinary Immunology, 10th Edition

dendritic cells can extend their processes into the intestinal lumen
VetBooks.ir and sample the microbiota. These bacteria can persist within the

dendritic cells for several days while the cells carry them into the
mucosa and mesenteric lymph nodes and present them to B cells. In

addition, some bacteria are taken up by specialized antigen-
capturing M cells, penetrate the Peyer's patches, and become
resident within the tissues. Although most of these invading
bacteria are killed by macrophages, some are also presented to B

cells. The B cells produce IgA, which may modify the composition
of the microbiota and block further mucosal penetration. Bacteria
are thus prevented from breaching the mucosal barrier by the
ongoing IgA response, and the mesenteric lymph nodes form an

additional barrier that prevents the commensals from reaching the
systemic immune system. It is likely that a similar local IgA
response occurs against food antigens. (The role of IgA is discussed
in detail in Chapter 22.)

T Cell Functions

The intestinal immune system must be relatively unresponsive to
the microbiota while still being able to respond to potential
pathogens. Thus it must be highly regulated. The key to successful

accommodation with the intestinal microbiota depends on the
body's ability to control inflammation in the gut wall. This is
achieved by a maintaining a balance between proinflammatory

Th17 cells and antiinflammatory Treg cells (Fig. 21.9).

670