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're going to have 50 years later," said Mark Davis, from Stanford University's Institute for Immunity, Transplantation and Infection.
But while genomic variation clearly plays a key role in some diseases, he said, the immune system has to be tremendously adaptable in order to cope with unpredictable episodes of infection, injury and tumour formation.
To determine nature's and nurture's relative contributions, Davis and his colleagues turned to a century-old method of teasing apart environmental and hereditary influences.
They compared pairs of monozygotic twins - best known as "identical" - and of dizygotic, or fraternal, twins.
The researchers recruited 78 monozygotic-twin pairs and 27 pairs of dizygotic twins. They drew blood from both members of each twin pair on three separate visits.
The team then applied sophisticated laboratory methods to the blood samples to measure more than 200 distinct immune-system components and activities.
This environmental dominance was more pronounced in older identical twins (age 60 and up) than in younger twins (under age 20).
The study was published in the journal Cell.

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