By Dr. Larry Chiaramonte
One subset of the hygiene hypothesis is called the “farm effect.” It refers to the lower incidence of allergies among rural populations such as the Amish than among city dwellers. The thought is that exposure to a greater variety of microbes from hanging around the barn jumpstarts immune activity in the direction of healthy acquired immunity rather than towards allergies. Given the fact that conventional wisdom in allergic disease shifts radically from time to time, it should surprise no one that this, too, needs some updating.
A new study called “Effects of early-life exposure to allergens and bacteria on recurrent wheeze and atopy in urban children”, by Susan V. Lynch, PhD et al tells us
1] Inner-city babies up to the age of one in homes with high levels of cockroach, mouse, and, to a lesser extent, cat allergen had less recurrent wheeze at age three. “This association was strongest for allergen levels in the first-year dust samples, suggesting that the first few months of life is a critical time period in childhood allergic disease development.” House dust mite and dog exposure during the first year of life did not have this suppressive effect on the production of IGE.
2] Reduced exposure to a variety of bacteria, as well as specific bacteria, was significantly associated with the development of allergies with wheezing or allergies alone. Some of the bacterial taxa were related to fewer allergies and less asthma in the long run. These protective bacteria were primarily members of the Bacteroidetes and Firmicutes phyla, along with some others, which were positively associated with mouse and cockroach allergen levels; what livestock do for farm kids, these yucky pests may do for inner-city ones.
3] Finally, combined analysis of exposure to both allergens and bacteria revealed that the children who neither wheezed nor sneezed had the highest first-year exposure to allergens and the protective bacterial species identified in the study. To the authors’ knowledge, this is the first scientific report of exposure to high levels of allergens combined with an environment rich in specific bacterial families as having such a protective effect.
This is a welcome study, which opens up a new front in understanding and preventing allergic disease. Not all antigens are created equal. Some like those from the parasite ascaris are allergens because they induce the formation of allergy-producing IgE even in adults. Conversely, under certain circumstances some allergens surely suppress the formation of IgE, and logically, age should be a factor. Newborns cannot make the protective antibody IgG in response to an antigen. They first respond by producing IgM. This IgM response does not produce an immune memory where a second exposure to an antigen is quicker and greater [amnestic response]. The production of IgG does create a potential for an amnestic response. Infants gain the ability to produce IgG somewhere between three and six months.
This study offers the chance to find the Holy Grail of allergy. How to prevent the rising increase in atopic disease is ultimately much more appealing to me as an allergist than treating them. Unfortunately, this comes closer to the end of my career rather than the beginning. I spend my time coaching families to clean their homes more diligently in order to avoid triggers for allergies they already have, not managing their hygiene to prevent them.
Regardless, it will be a long time before these insights find their way into actively managing the tendency to these diseases. Still, I think it’s time for some bench research on infant experimental animals that have a similar developmental sequence to humans with their immune system. Ethical considerations would limit the ability to manipulate the human immune system in infants. Can mice be sensitized to their own droppings? Stay tuned.