Close friends are the people with whom we share our deepest desires and self-doubts. But friendship may do more than just cheer us up: It may also be a kind of behavioral vaccine that can inoculate us against illness.

Growing scientific evidence indicates that friendships and extensive social networks can lower blood pressure; reduce the risk of diabetes and heart disease; ward off depression; and make us less susceptible to the ravages of old age. The Harvard Nurses’ Health Study, for instance, found that the more friends women had, the less likely they were to become physically debilitated as they grew older.

Fortunately, the impulse to reach out to others when we’re troubled—the female penchant to “tend and befriend”—seems to be hardwired into our brains, according to research from UCLA. Women have a third possible reaction to stress besides the well-known fight or flight responses. Intense pressure can trigger the release of oxytocin, a calming hormone that drives us to seek the company of others. The companionship, in turn, leads to the release of more oxytocin, which creates a sense of well-being, says Waguih William IsHak, MD, a psychiatrist at Cedars-Sinai Medical Center in Los Angeles.

And it’s the stress-relieving aspect of friendship that may play a big role in helping women become healthier, says David Spiegel, MD, a psychiatrist at Stanford University School of Medicine. In a 2000 study, Spiegel found that disruptions in the normal pattern of the stress hormone cortisol predicted earlier mortality in breast cancer patients. On the other hand, “Psychosocial support helps women handle stress,” Spiegel says. That means letting off steam with our closest intimates could be as health-promoting as a potent prescription medicine.