Is playing in the dirt good for kids' immune systems?

As a child, you may have been told that playing outside in the dirt is good for you because it strengthens your immune system. But is there any evidence to back this claim? 

In short, yes — several studies suggest that early exposure to dirt may reduce kids' risk of developing allergies and autoimmune conditions. In other words, it may guard against conditions in which the immune system unhelpfully reacts to allergens or to the body's tissues.

As a child's immune system develops in the early years of life, the army of protective cells within the body have to learn how to distinguish between the body's own cells and foreign substances that are either harmless or disease-causing, such as bacteria and viruses. It must learn to spot the disease-causing pathogens in order to attack them. 

Related: What do some allergies last a lifetime? Newly described immune cells to blame

It turns out that the molecular signals that drive the expansion of this regulatory arm of the immune system mainly come from microbes in the gut, Graham Rook, a professor emeritus of medical microbiology at University College London, told Live Science. This collection of microbes is called the "gut microbiome" and is essential to our health. For instance, some of these microbes help produce vitamins that we need to live, and they help us digest our food. 

The first year of life is crucial to the microbiome's development. Babies receive bacteria as they pass through the birth canal, if they're born vaginally, and from milk if they're breastfed. As children grow, they are steadily exposed to microbes from a wide array of sources. 

A theory called the "old-friends hypothesis" suggests that, the greater the range of microbes we're exposed to in early childhood, the more diverse our microbiomes will be and thus the better our immune systems will recognize friend from foe. The term "old friends" refers to helpful, or "commensal," microbes that live on and in the body without harming a person's Health.