Some scientists have proposed that the human immune system needs something to keep it busy in early childhood. Without enough microbes to fight, they say, idle or improperly educated immune cells may make mischief later by inducing reactions against substances that don't merit the effort. This still controversial notion, called the hygiene hypothesis, holds that such children are prone to develop hypersensitivity later—in the form of allergies or asthma (SN: 8/14/99, p. 108: http://www.sciencenews.org/sn_arc99/8_14_99/bob2.htm).

The most recent evidence in this debate shows that young infants kept out of day care and having no more than one older sibling are significantly more likely to develop asthma than are babies who have greater exposure to other children.

Starting in the early 1980s, epidemiologist Anne L. Wright of the University of Arizona College of Medicine in Tucson and her colleagues tracked 1,035 infants from birth. Parents brought the children in for regular checkups and reported any wheezing episodes or asthma diagnoses.

During the study, asthma showed up in 21 percent of children with one or no older siblings and no daycare exposure before age 6 months. Only 12 percent of children who spent time in day care before age 6 months or who had at least two older siblings developed the condition, the team reports in the Aug. 24 New England Journal of Medicine (NEJM).

Blood tests showed that immunoglobulin E, an antibody abundant in people with asthma, was plentiful in 35 percent of the less exposed children but in only 27 percent of the other group. The researchers also did skin tests to gauge sensitivity to alternaria, an allergy-causing mold in the Tucson area. They found that 25 percent of the children who had been less exposed to other kids reacted to it, compared with 17 percent of the children who had been in early day care or had multiple older siblings.

By age 2, the children with greater exposure to kids as infants had experienced more wheezing episodes than the others. By age 8, however, this group reported less wheezing than the other children. In the preschool years, wheezing is typically caused by infections, but in school-age children, it more often results from allergies or asthma, Wright notes.

Several previous studies showed that babies in day care contract more infections on average than stay-at-home children do. Another study found that infants placed in day care had fewer allergies later than did children who entered day care at an older age. Scientists consider asthma a subset of allergy.

The hygiene hypothesis holds that exposure to microbes provides an infant's immune system with clear signals for proper development, Wright says. Several recent studies have bolstered that contention, suggesting that parents in industrialized societies, where asthma rates have soared, permit children too little contact with germs.

"We didn't evolve in some sort of sterile capsule. We evolved in a hostile environment," says Julian M. Hopkin of the University of Wales in Swansea. "This [study] strengthens the idea that . . . microbial exposure may limit asthma."

"For those of us who share the furtive guilt of having left marginally ill toddlers at day care, these findings also offer a sense of relief," says Sandra C. Christiansen of the Scripps Research Institute in La Jolla, Calif., in the same NEJM issue. "We were perhaps only doing our part to restore the [immune-system] balance and to stem the rising tide of asthma and allergic disease."

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