http://www.scientificamerican.com/article/your-immune-system-is-made-not-born/
The whole article is quite interesting, too long to copy all of it
In one of the most comprehensive analyses of immune function performed to date, researchers analyzed blood samples from 105 sets of healthy twins. They measured immune cell populations and their chemical messengers—204 parameters in all—before and after participants received a flu shot. Differences in three fourths of these parameters depended less on genetics than on environmental factors, such as diet and prior infections. Genetics had almost no effect on how well individuals responded to the flu vaccine, judged by antibodies produced against the injected material. And among identical twin siblings, who have the same genome, immune system features differed more strikingly within older twin pairs than in younger sets. The findings, published January 15 in Cell, argue that life habits and experiences shape our body’s defenses more than the DNA passed down from our parents.
Although prior twin studies had hinted that nonheritable factors contribute to some autoimmune disorders, such as multiple sclerosis, the recent analysis was one of the first to quantify genetic and environmental effects on the general immune system. “We were surprised by the degree of environmental influence on so many components,”
... One finding was particularly striking. A single environmental factor—a past infection with common cytomegalovirus—affected 58 percent of the tested parameters. Whereas the results don’t show whether these changes produce an overall stronger or weaker immune response, they do indicate “cytomegalovirus has a really profound effect,” Davis says. The Epstein–Barr virus, another microbe that frequently infects people, had no such effect.
... scientists are uncertain as to why the Epstein-Barr virus, which also infects most people and lingers in the body, doesn’t trigger a big ongoing immune response like cytomegalovirus does. It could be that the Epstein-Barr virus primarily infects B cells whereas cytomegalovirus can hide in a variety of cell types. Davis says his team is taking a closer look at Epstein-Barr virus’ effects on the twins’ immune parameters and plans to report the findings soon
The whole article is quite interesting, too long to copy all of it