With modern plumbing and hygiene, the number of nasty microbes
we humans are exposed to has plummeted, while the rate of
autoimmune diseases and allergies has shot up. Are those
related? Proponents of the hygiene hypothesis think so: our
immune system is supposed to develop by encountering microbes,
so being too clean throws it out of whack as the immune system
overreacts to minor insults. A new study found that mice raised
germ-free had especially high numbers of invariant natural
killer T cells (iNKT) in their colons and lungs—the mouse
versions of inflammatory bowel disease and asthma, respectively.
Most evidence supporting the hygiene hypothesis has just been in
observed correlations, so this research that identifies a
plausible molecular mechanism is good evidence for how over-
cleanliness might cause immune dysfunction. The researchers
homed in on a signaling molecule called CXCL16, which is
associated with the build-up of iNKT cells. iNKT cells identify
what cells that seem to be pathogens—what might be