( Answering one of the oldest questions i...)Why doesn't the immune system attack the small intestine?

In a study in the February issue of Nature Immunology, and which is currently available on the journal's Web site as an advanced online publication, investigators led by Shannon Turley, PhD, of Dana-Farber identify an unlikely group of peacemakers: lymph node cells that instruct key immune system cells to leave healthy tissue alone. The finding, which illuminates a previously unknown corner of the human immune system, may lead to new forms of treatment for autoimmune diseases such as Type 1 diabetes and multiple sclerosis.

"We've discovered that cells not generally thought of as part of the immune system actually play an important role in protecting the intestine from immune system attack," says Turley. "Because the cells are found in lymph nodes throughout the body, they may offer a way of suppressing a variety of autoimmune diseases," which result from immune system assault on healthy tissue.

The immune system distinguishes between normal and foreign agents by small proteins, called antigens, on the cell surface. In parts of the body, such as the pancreas, that are sheltered from the outside environment, cells known as dendritic cells display the antigens of their normal neighbors in a way that puts the immune system "at ease." By reading those antigens without being on alert, the immune system's T cells learn that such cells are off-limits to attack.

For years, scientists have wondered whether the same mechanism is at work in tissues that come in regular contact with bacteria and other microbial organisms. The small intestine, for example, which absorbs essential nutrients from food and drink and protects the body from invasive microbes,
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Source:Dana-Farber Cancer Institute