Washington, Jan 19 (IANS): After studying twins, researchers from Stanford University show that our environment, more than our heredity, plays the key role in determining the state of our immune system.

This is especially true as we age, the study indicates.

"The idea in some circles has been that if you sequence someone's genome, you can tell what diseases they are going to have 50 years later," said Mark Davis, director of Stanford's institute for immunity, transplantation and infection.

But while genomic variation clearly plays a key role in some diseases, "the immune system has to be tremendously adaptable in order to cope with unpredictable episodes of infection, injury and tumour formation".

"The immune system has to think on its feet," Davis noted.

To determine this, Davis and his colleagues compared pairs of monozygotic twins - best known to most of us as "identical" - and of dizygotic or fraternal twins.

Monozygotic twins inherit the same genome and remain almost 100 percent genetically identical.

Dizygotic twins are no more alike genetically than regular siblings, on average sharing 50 percent of their genes.

For the new study, the team recruited 78 monozygotic-twin pairs and 27 pairs of dizygotic twins from the registry of twins for research purposes at Stanford.

Scientists found that in three-quarters of the blood measurements, nonheritable influences - such as previous microbial or toxic exposures, vaccinations, diet and dental hygiene - trumped heritable ones when it came to accounting for differences within a pair of twins.

This environmental dominance was more pronounced in older identical twins (age 60 and above) than in younger twins (under age 20).

In a striking example of the immune system's plasticity, the scientists found that the presence or absence of a single chronic viral infection could have a massive effect on the system's composition and responsiveness.

"Nonheritable influences, particularly microbes, seem to play a huge role in driving immune variation," Davis pointed out.

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," the authors concluded.

The study is forthcoming in the journal Cell.