The study corroborates other recent work that supports the "hygiene hypothesis" -- speculation that early exposure to infectious or inflammatory agents causes changes in babies' immune systems that reduce their risk of developing allergy-related conditions later in life. Various studies have looked at the relationship between endotoxin and allergies, but this is the first U.S. study to look at the effects of endotoxin exposure on eczema, one of the most common allergic diseases of infancy.
As part of the ongoing, Boston-based Home Allergens and Asthma Study, the researchers followed nearly 500 infants living in the Boston metro area beginning at 2 to 3 months of age. They collected dust samples from the living rooms of 400 of them and analyzed the samples for endotoxin, a component of the cell walls of various bacteria. All the babies had a parent with allergies or asthma.
The more endotoxin in the home, the less likely babies were to be diagnosed with eczema during their first year of life. When endotoxin concentrations were classified into four levels, the risk for eczema was about 25 percent lower with each increase in endotoxin level, after adjustment for other eczema-related variables. Infants whose homes had a dog also were less likely to develop eczema, but this relationship weakened after adjustment for endotoxin exposure. Eczema risk was increased in infants whose fathers had a history of eczema, or whose mothers were sensitive to at least one allergen.
Cases of eczema have increased two- to three-fold in industrialized countries since World War II, a pattern similar to that seen for asthma and other allergic conditions. According to the hygiene hypothesis, today's cleaner, more germ-free environments may deprive babies' developing immune systems of chances to practice fighting off microbes; as a result, their immune systems veer toward an allergic type of response, mistakenly attacking harmless substances. Supporters of this still-controversial idea note that children who live on farms, grow up with pets, come from large families, or start day care in early infancy are less prone to allergies and asthma.
Phipatanakul does not advise parents to buy pets, stop cleaning their homes, place their infants in daycare, or do anything else to protect them from eczema.
"We now know that there are things in the environment that may be important in eczema," she says. "But you need lots of studies to be able to come up with recommendations."
Phipatanakul notes that the mechanisms of allergies are very complicated, and that it's not clear what endotoxin does to the immune system. In fact, a number of studies have linked endotoxin exposure with wheezing and airway inflammation in established asthmatics.
The research was funded by the National Institutes of Health.
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