Though it doesn’t pay to dwell on the fact, most homes are full of Dermatophagoides pteronyssinus, commonly known as dust mites. Clambering through forests of carpet pile and steamy mattress-pad tropics, these blind but hardy arachnids feed on bits of dead skin that flake off the mammalian residents of the household. Researchers have known for years that the mites can cause allergic asthma attacks, but evidence of just how they do it has been lacking. Immunologist Colin Hewitt of the University of Leicester in England has now found at least part of the answer.
An enzyme called Der p 1, found on the surface of mite fecal pellets--mites use it to digest proteins--triggers the attacks. One mite can produce about 40 pellets a day, which people inhale when dust gets stirred up. In the human respiratory tract, an antibody called IgE attached to a carrier cell recognizes Der p 1 and binds to it. The carrier cell then produces histamines, sometimes enough to cause swelling, itching, and ultimately sneezing--responses meant to expel the foreign body.
In an allergic asthmatic response, however, these defense mechanisms run amok--the symptoms continue because the normal controls have somehow been disrupted. Hewitt and his colleagues have discovered how this can happen.
They’ve found that Der p 1 snips off a key receptor protein on another immune cell called a B cell that produces IgE. Normally when IgE binds to this receptor, the B cell stops producing more IgE, which in turn cuts off the production of histamines. If you clip off that receptor, says Hewitt, there’s nothing telling the cell to stop making IgE. It’s blind to the fact that it’s made too much, so it keeps on making more.
A layer of molecules called antiproteases lines our lungs and defends against protein snippers like Der p 1. But occasionally they sneak through. It may be that people who suffer intense allergic reactions don’t have enough of these protective molecules and that the intensity of their response is caused by the greater amount of Der p 1 getting through. Hewitt points out that some components of cigarette smoke are known to damage antiproteases. Children under the age of two, with still-developing immune systems, are particularly susceptible to developing asthma. So if you have dust mites and cigarette smoke in the same environment, Hewitt says, and you’re exposed to it for a good deal of the day, as a small child would be, then there’s a potential mechanism by which you get these two factors leading to allergy.