Page 667 - Veterinary Immunology, 10th Edition

VetBooks.ir Immune Responses to the Microbiota

For many years, it was believed that the role of the immune system
was simply to ensure the complete exclusion of all invading

microbes by distinguishing between self and not-self and
eliminating foreign antigens. We now know, however, that that
decision alone is insufficient to ensure health. The immune system
must also determine the degree of threat posed by the microbes it
encounters and adjust its response accordingly. It must maintain

tolerance to microbiota or food antigens while, at the same time, be
highly responsive to invading pathogens. This discrimination is
determined in part by the way in which enteric antigens are

processed and the behavior of enterocytes. It is also determined by
intestinal T and B cells as well as by the gut microenvironment.
The presence of the intestinal microbiota must either be tolerated
or ignored if an animal is to remain healthy. An animal cannot
afford to act aggressively toward its own microbiota. The presence

of all these bacterial products has the potential to trigger massive
acute inflammation, but this inflammation must not happen unless
necessary for the defense of the body.

Enterocytes

The intestinal epithelium is not simply a barrier. It is a highly
responsive tissue that employs innate and adaptive immune cells to
restrain the microbiota without triggering unnecessary

inflammation, but is always ready to activate more potent defensive
responses should the need arise. Enterocytes interact with the
intestinal microbiota. They produce many peptides that kill or
inactivate bacteria and as a result shape its composition.

Enterocytes block access of intact antigens to the lamina propria.
They ensure that a balance exists between inflammation and
tolerance. They secrete and respond to regulatory cytokines. They
display antigens to dendritic cells. Within the epithelium and the

underlying lamina propria are IELs that upon appropriate
stimulation by microbiota-induced IL-1 or IL-23 can regulate their
differentiation into effector or regulatory cells.

667