Why doesn’t our immune system attack tissues harboring bacteria in our small intestine?

SpiritIndia.com: Answering one of the oldest questions in human physiology, researchers at Dana-Farber Cancer Institute have discovered why the body's immune system -- perpetually on guard against foreign microbes like bacteria -- doesn't attack tissues in the small intestine that harbor millions of bacteria cells.

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 scelerosis.

"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.

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