We're Outnumbered 10 to 1

In today's WSJ: Immune System's Double Duty:

We're not only human after all.

New genetic technology has revealed that the human body swarms with more benign bacteria than ever imagined -- more than 2,000 species, whose cells outnumber the body's own by a 10-1 ratio. But that isn't such a bad thing. Many of these bacteria are needed for tasks such as the digestion of nutrients and the development of organs.

That such a vast microbial community is permitted to thrive in our bodies upends the notion that the immune system's role is simply to attack invading microbes. A growing number of biologists believe that, in addition to its protective role, the immune system acts as a master regulator of our microbial menagerie, working "to maintain communities of bacteria in balance," said Margaret McFall-Ngai, a University of Wisconsin biology and immunology professor. She is a creator of this hypothesis, which conceptualizes the immune system as keeping each species in its proper niche and quantity.

If confirmed, this hypothesis could have wide-ranging consequences for medicine because a growing number of health problems, from inflammatory-bowel disease to obesity, have been linked to bacterial communities out of balance, as opposed to a lone pathogen...