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
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