Let them eat dirt

A new study in Science supports the idea that exposure to germs in childhood helps develop the immune system and thereby prevent allergies and other immune-related diseases such as asthma and colitis later on in life.

The "hygiene hypothesis" proposes that early childhood exposure to microbes increases susceptibility to certain diseases by suppressing development of the immune system. The new study not only supports this idea, but may also explain the whys and hows.

However, they investigated mice, so it does not necessarily mean the same results would occur in humans. Researchers bred "germ-free" (GF) mice, that are bred in a sterile environment, without exposure to microbes, and specific-pathogen-free (SPF) mice raised in a normal laboratory environment, to develop forms of asthma and inflammatory bowel disease (IBD) and compared their immune systems. They found that the GF mice had more invariant natural killer T (iNKT) cells in their lungs and bowel, and developed more severe disease symptoms:

When they exposed GF mice to germs in their first few weeks of life, they did not develop high levels of iNKT cells, and they did not develop the more severe symptoms seen in those kept germ-free. The early-life exposure proved to be long-lasting.

This shows the critical importance of proper immune conditioning by microbes during the earliest periods of life and research in humans needs to proceed.