Being lonely makes you feel like crap emotionally, but because social isolation is also a risk factor for a whole bunch of illnesses, it can make you feel bad physically, too. Exactly how that happens has been a bit of a mystery, but now researchers are beginning to understand the way loneliness affects some of our immune system's most important defenses.In a new study , published this week in the Proceedings of the National Academy of Sciences, researchers looked at the way being socially isolated can affect our DNA. They examined cells involved in the immune system in blood and urine samples from 141 participants who were part of another decade-long study . All participants were also given surveys that measured their levels of loneliness every year.The results showed that participants who scored higher on the loneliness survey also showed a series of changes related to the expression of genes that play a role in the immune system. Lonelier participants had higher levels of the neurotransmitter norepinephrine, which is a part of our fight-or-flight responses to stress and can change the amount of white blood cells we produce. In the study, the DNA of those white blood cells showed an increase in the expression of genes that play a role in inflammation, and a decrease in the expression of genes that help those cells fend off bacterial and viral infections (such as colds ).Looking at similar biological markers in macaque monkeys, the researchers found a key difference between the production of white blood cells in those who are socially isolated and those who aren't: Lonelier monkeys had more monocytes — "immature" white blood cells — than normal. So, the authors suggest that loneliness causes stress, which in turn causes this shift in the production of the immune system's white blood cells, making us (and the monkeys) more susceptible to illness.As we've written before , other research has shown a pretty strong connection between social isolation — long-term loneliness, not just a little FOMO on a Friday night — and some serious illnesses, including cancer and heart disease. Chronic stress similarly weakens the immune system , making you even more vulnerable to sickness; all this suggests that being lonely for an extended period of time may really be a type of chronic stress. But we also know that having a network of friends and/or family can protect against those effects. So even if they may cause you stress sometimes, having people in your life can still be good for your health — and theirs.