Researchers at the Brigham and Women's Hospital (BWH) in Boston led the study and published their results in the March issue of Science magazine. Using the "hygiene hypothesis," the team says research shows a lack of exposure to microbes at an early childhood age increases susceptibility to some diseases because the lack of exposure suppresses the body's immune system. The study does more than just support the notion, it also may explain the whys and hows of the process.
Researchers warned, however, that their research was conducted on mice, not humans. Still, the results seemed to indicate that you have to trigger the immune system with the introduction of germs in order for it to develop fully.
How it works
The research team, led by co-authors Dr. Richard Blumberg, chief for the BWH Division of Gastroenterology, Hepatology and Endoscopy, and Dr. Dennis Kasper, director of BWH Channing Laboratory, studied "germ-free" mice bred in a sterile environment without exposure to microbes, as well as specific-pathogen-free mice that were raised in a normal laboratory environment.
The mice were bred to develop forms of asthma and inflammatory bowel disease, in which their immune systems were then compared.
The team found that the germ-free mice had more invariant natural killer T cells in their lungs and bowel, and developed more severe disease symptoms.
"[... W]e show that, in germ-free (GF) mice, invariant natural killer T (iNKT) cells accumulate in the colonic lamina propria and lung, resulting in increased morbidity in models of IBD and allergic asthma compared to specific pathogen-free (SPF) mice," Blumberg and Kasper wrote.
The researchers also found that when they exposed germ-free mice to mice with germs in their first few weeks of life, they didn't develop high levels of invariant natural killer T cells. Also, they didn't develop the more severe symptoms seen in those mice kept germ-free. And, they discovered, germ-free mice with early-life exposure to microbes developed long-term disease protection.
"These studies show the critical importance of proper immune conditioning by microbes during the earliest periods of life," Blumberg told reporters. "Also now knowing a potential mechanism will allow scientists to potentially identify the microbial factors important in determining protection from allergic and autoimmune diseases later in life."
The origin of the hygiene hypothesis dates back to 1989, when a researcher named D.P. Strachen first proposed it in the British Medical Journal. His theory asserts that "the reduction of early childhood infectious disease, due to widespread vaccination of children or increased use of antibiotics, has led to an increased prevalence of allergic diseases," writes Lori Baker Schena for the University of Southern California Health Magazine.
Meanwhile, Strachen found that there were lower incidents of asthma and allergy in large families.
"It seems that parents these days are a lot more worried about minor infections in their kids," Ronald M. Ferdman, M.D., assistant professor of clinical pediatrics at the Keck School of Medicine and attending physician in the division of clinical immunology and allergy at Children's Hospital Los Angeles, told the magazine.
"Nobody would say that you should purposely expose your child to infections. On the other hand, there is no need to keep children in a bubble," he added. "In America, there is no risk of exposing children to malaria or some horrendous disease. But parents do get upset if their child comes down with a cold."
And it's that paranoia that, in all actuality, could be making our children more susceptible to immune-system deficiencies.