A recent Stanford University twins study concluded that environmental forces are more influential than genes when it comes to determining the make-up of an individual’s immune system. The study, led by Stanford immunologist Mark Davis, compared 210 identical and fraternal twins between 8 and 82 years old.

After analyzing blood samples that tracked 200+ immune system parameters including cell population frequencies, cytokine responses, and serum proteins, researchers found that in three-quarters of the measurements, differences between pairs of twins were strongly linked to non-heritable factors such as vaccinations, diet, and previous infections. Notably, a key environmental cause of immune system variation was the presence cytomegalovirus ("CMV").

CMV is a common chronic infection affecting three in five Americans, and while usually harmless, CMV can pose a health threat to immune-compromised individuals and babies infected congenitally. Once CMV is in a person's body, it remains for life.The Stanford team examined 16 pairs of identical twins where only one was infected with CMV, and found significant variance in nearly 60% of the parameters studied. Specifically, CMV-discordant twins showed less similarities for many immune cell frequencies such as effector CD8+ and gamma-delta T cells. Similar results were seen in signaling responses to IL-10 and IL-6 stimulation, as well as in the concentrations of these same cytokines in serum.

Overall, the study confirmed that CMV’s influence was widespread, affecting 58% of the 204 immune system measurements. These findings demonstrate how a single chronic viral infection can dramatically alter the immune system’s composition and responsiveness.

The study’s authors concluded that when the immune system is tasked with a lifelong need to control a viral infection like CMV, it may create a broad shift in the magnitude and complexity of many cell subsets. Research suggests that as many as 10% of all T cells in CMV positive individuals may actively restrain viral replication and prevent disease.