The state of our immune systems vary a lot from person to person. You might react to flu germs and even the vaccine differently than your officemate, for example. Do these dissimilarities reflect genetic differences or something else? Now, researchers working with 210 twins reveal that immune variation is driven largely by the environment, rather than heredity. This is especially true as we age. The findings are published in Cell this week.
“The idea in some circles has been that if you sequence someone’s genome, you can tell what diseases they’re going have 50 years later,” Stanford’s Mark Davis says in a news release. Our genes play a key role in some diseases, of course, but the immune system must be highly adaptable to cope with infections, injuries, and tumor formation, which can all occur unpredictably, he adds. “The immune system has to think on its feet.”
To understand our different immune responses, Davis and colleagues turned to a classic method of teasing apart the contributions of nature versus nurture: compare pairs of identical twins and pairs of fraternal twins. The former inherit the same genome from their parents, while the latter share half of their genes on average, like siblings who didn’t share a womb.
For this study, the team recruited 78 pairs of identical twins and 27 pairs of fraternal twins. They took blood samples on three separate visits and measured more than 200 immune-related parameters. “What we found was that in most cases,” Davis says, “there is little or no genetic influence at work, and most likely the environment and your exposure to innumerable microbes is the major driver.”
In three-fourths of the measurements, the team found that non-heritable factors—including diet, dental hygiene, vaccines, previous infections, and exposure to toxins—determined the individual variation in the immune system within twin pairs. After receiving a flu vaccine, for instance, twins produced different amounts of antibodies.
Additionally, the team found that a previous infection by a pathogen called cytomegalovirus massively affects the composition and responsiveness of the immune system. In 16 of the identical twin pairs, one twin had been exposed to cytomegalovirus while the other hadn’t. Its presence in one twin and absence in the other made a big difference in nearly 60 percent of the parameters they measured.
Furthermore, the dominance of environmental factors becomes even more obvious with age when comparing twins aged 60 and up with twins under 20. “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,” Davis explains. “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.”