A twin study has shown that the majority of variation in immunity between individuals is due to non-genetic factors.
The findings, published in Cell, suggest that the environment dominates when it comes to shaping our immune profiles.
'When you examine people's immune systems, you often find tremendous differences between them. So we wondered whether this reflects underlying genetic differences or something else,' explained Professor Mark Davis, one of the study authors from Stanford University, California.
'But what we found was that in most cases, including the reaction to a standard influenza vaccine and other types of immune responsiveness, there is little or no genetic influence at work, and most likely the environment and your exposure to innumerable microbes is the major driver.'
The research team compared 78 pairs of identical twins and 27 pairs of non-identical twins who were aged between eight and 82 years. This classic study method allowed the researchers to tease apart the hereditary and environmental influences on immunity as all twin pairs can be assumed to have shared the same childhood environment, while only the identical twins are genetically the same.
Using blood samples, the researchers measured more than 200 immune system components and activities. They found that for three-quarters of these, non-heritable influences - which could include things such as previous microbial or toxic exposures, vaccinations, diet and dental hygiene - were more important than heritable factors when it came to accounting for differences between pairs of twins.
The team also noted that the effect of the environment was more pronounced as twins got older, suggesting the effect accumulates over time.
Additionally, as some of the participants had also received flu vaccination as part of another study at the same university, the researchers were able to examine the levels of antibodies produced in reaction to the vaccine. They found that, again, non-heritable factors were largely responsible, conflicting with previous reports suggesting a large genetic component to vaccine responsiveness.
A further finding was that in identical twin pairs where one had been exposed to cytomegalovirus, a common chronic infection, and the other had not nearly 60 percent of immune features were affected, demonstrating that a single non-heritable factor can have a major effect on the immune system's composition.
'Nonheritable influences, particularly microbes, seem to play a huge role in driving immune variation,' said Professor 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. 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.'