Scientists have surprising new evidence that the immune system, best known for protecting the body against pathogens, also plays a key role in the brain's ability to grow new neurons in adulthood.

If true, boosting the immune system may be one way to protect against age-associated learning and memory problems, said Michal Schwartz, lead author of a paper on the research published this month in Nature Neuroscience.

Schwartz, a professor of neuroimmunology at the Weizmann Institute of Science in Israel, and colleagues discovered that immune cells in the blood called T-lymphocytes are critical to brain cell proliferation. It is an unusual autoimmune response, she said.

T-lymphocytes normally enter the brain to patrol for signs of infection. But scientists have discovered that these immune cells recognize a normal brain protein as foreign and mount an immune response by pumping out activated microglia, cells that produce inflammation. These microglia support the birth of new neurons in these brain regions.

Schwartz came to this idea when she identified large amounts of activated microglia in the brain regions that give rise to new neurons in the adult brain. One of these regions, the hippocampus, is the seat of learning and memory

She studied animals born without an immune system and found production of new brain cells in adulthood was severely limited. She repeated the same study in animals lacking the ability to make T-lymphocytes. Again, the animals had trouble growing new neurons.

As she expected, stimulating the production of T-lymphocytes led to the birth of even more neurons than expected.

In both cases, the animals showed marked differences in their ability to learn. Those without the T-lymphocyte autoimmunity failed to learn a simple water maze task. Animals with large numbers of T-lymphocytes learned to navigate the water maze much faster than even normal animals.

"It's a novel idea that's backed up by strong data," added Michael Chopp, vice chairman of neurology at the Henry Ford Hospital in Detroit. Schwartz suspects that similar immune processes are at work in the human brain and that an aging immune system could set the stage for dementia.