"A healthy human immune system continually adapts to its encounters with hostile pathogens, friendly gut microbes, nutritional components and more, overshadowing the influences of most heritable factors," says lead author Dr. Mark Davis, director of Stanford's Institute for Immunity, Transplantation and Infection.
Twins make excellent subjects in this type of research, according to the researchers, because under typical circumstances they share the same environment both in childhood and in utero.
They recruited 78 pairs of identical twins -- defined as such because they inherit the same genome -- and 27 pairs of fraternal twins, who have different traits due to different genetic make-up.
Blood was drawn from all participants on three separate visits to the laboratory and the research team measured over 200 distinct components and activities of their immune systems.
As they tallied up the differences in the twins, the research team found that three quarters of those measurements concerned things that influence the immune system that are not inheritable.
Examples of this include exposure to microbes or toxins, vaccinations, diet and dental hygiene, and these sorts of environmental influences that account for differences in the twins' immune systems were most common in those over 60 years of age.
The sets of twins under 20 years old showed the least of these differences, indicating that the environment's influence over the immune system is increasingly strong as we age.
"Nonheritable influences, particularly microbes, seem to play a huge role in driving immune variation," says Dr. Davis. "At least for the first 20 or so years of your life, when your immune system is maturing, this amazing system appears able to adapt to wildly different environmental conditions."
Dr. Davis and his team concluded their study, which was published in the journal Cell, by saying that carrying a just one chronic viral infection could shake up the immune system in a big way.