If you're like me, you think that it's healthy for kids to play in the dirt and wrestle with dogs and suffer through a cold now and then. I think of it as giving your immune system a workout. That way, when it runs into something like the H1N1 flu virus, it's got some muscles built up--in the form of lots of B and T cells and antibodies. That's the hope.
The immune system's more complicated than that, but the basic theory that it's good for kids to eat dirt is known as the "hygiene hypothesis." The subtlety, according to the theory, is that a lack of exposure to microbes prevents the immune system from developing a tolerance for certain antigens, and leaves the body more likely to develop auto-immune diseases. Last week the hygiene hypothesis got some support in the form of laboratory experiments conducted on mice by a team of researchers in Boston and Kiel, Germany.
In their paper, published in Science [nice news story in Nature too], the scientists explain how they worked with two types of mice--one, "germ-free," devoid of any bacteria or protozoa or viruses (except viruses already integrated into their DNA), and the other, "specific-pathogen-free," which they don't describe very well but which you assume have well-established symbiotic gut microbes but aren't infected with any diseases. The germ-free mice, the authors find, have increased numbers (compared to the SPF mice) of white blood cells called "invariant natural killer T cells," which can detect certain kinds of molecules associated with infection; cause inflammation when they're activated; and have been linked to both inflammatory bowel disease (IBD) and asthma.
The germ-free mice had increased numbers of NKT cells in just their colons and lungs, and were far more susceptible than the SPF mice to laboratory models of asthma and IBD.
But when the scientists exposed pregnant germ-free mother mice to the same living conditions as the SPF mice, so that they and their newborns could acquire a population of gut microbes, the thus-exposed "germ-free" baby mice went on to develop relatively normal populations of NKT cells, and were protected from getting asthma and IBD.
However, allowing adult germ-free mice to build their own gut flora did not protect those mice from asthma and IBD. So it seems there's a window in which young mice must be exposed to microbes--to play in the dirt, as it were--if they are to get any benefit.
Because this paper was published in Science--about as high-profile as it gets--I'm guessing that clear evidence supporting the hygiene hypothesis is sparse. What conclusion could you draw from this research?
No human child, excepting the Bubble Boy, is brought up in an environment remotely resembling that in which germ-free mice are maintained. We're all crawling with bugs inside and out; dirty whether we like it or not. But Western society is quite keen on taking antibiotics to treat bacterial infection. And most antibiotics are pretty indiscriminate in what they kill. That's why the pharmacist warns you that your digestion may be upset while you pop Amoxicillin. It clears out your intestines.
So giving very young children oral antibiotics unnecessarily could be doing them harm in the long run by depriving them of gut microbes during a critical window--and those kids could grow up more likely to develop asthma and IBD.
Does the hygiene hypothesis, and this research in particular, apply to eczema? I put this question to Jon Hanifin, MD, a dermatologist at Oregon Health Sciences University, and am hoping to hear back from him.
[Monday, March 26] And here are his replies:
Question 1 : is this research relevant to atopic dermatitis? (are natural killer cells involved in AD?)
JH: It would be pretty speculative to claim relevance to AD but the "hygiene hypothesis" has been suggested for all the atopic conditions for many years and maybe this model will provide some some basis for testing the hypothesis. NK cells have probably been studied in AD just like all the others but much of the work with any of the T cells can be misleading so it is best to step back and look at the larger picture.
Question 2: if so, is there anything usable for patients, extrapolating from the results; could it possibly be that infants in modern society/urban centers aren't being exposed early on to the "right" mix of/number of nonpathogenic microbes?
JH: Not much at this point--presumably infants get exposed to all the microbes parents carry. The idea that farm kids get a broader palette seemed to have some validity for respiratory allergies but not much for AD. The greater AD incidence in urban kids might reflect more barrier damage from use of irritating/drying skin care products.