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How your immune system does its job seems to depend more on your environment and the germs you encounter than on your genes, says new research that put twins to the test to find out.

After all, the immune system adapts throughout life to fight disease, said Stanford University immunologist Mark Davis, who led the work.

And while young children's immunity may be more influenced by what they inherit from mom and dad, Thursday's study showed genetic influences waned in adulthood.

"Experience counts more and more as you get older," said Davis, director of Stanford's Institute for Immunity, Transplantation and Infection.

Scientists know there is tremendous variation in how the immune systems of healthy people function. Davis asked if that's more a matter of nature or nurture, by comparing 78 pairs of twins with identical genetic makeups to 27 pairs of fraternal twins, who are no more alike genetically than any other siblings. Traits shared by the identical twins are more likely to be hereditary.

His team used blood samples from the twin pairs, who ranged...