Firdaus Dhabhar likes to film babies crying when they get their shots, but not for any sadistic reason. He believes that the wailing is a good sign. A Stanford University researcher who studies how stress changes the body, Dhabhar, along with his colleagues, has discovered that stressed-out laboratory mice exhibit more robust immune responses to vaccines than control groups of mice left in peace. Something similar happens to people. In a study of knee surgery patients, for example, Dhabhar found that the anxiety of their impending operations boosted the number of immune cells circulating in their blood. Such studies have convinced Dhabhar that stress does not entirely deserve its bad reputation and that in some situations it can actually improve health.
Dhabhar and his collaborators contrast the benefits of short-term stress with the consequences of chronic stress, which has long been known to suppress the immune system. Then again, chronic stress can also exacerbate allergies, asthma and autoimmune disorders in which the immune system is already overactive. So does stress excite or repress the immune system? Here is where things get frustratingly fuzzy, as they so often do in biology. It turns out that the answer hinges on the situation and the individual. A transient burst of stress tends to activate some parts of the immune system but not others; conversely, chronic stress generally stifles the entire immune system and can make it more likely to attack benign tissues.
In the knee surgery study, patients' immune systems did not all respond equally to anticipation of the operation. Some people showed an agile, adaptive response: the number of immune cells in their bloodstream peaked in the days before the operation, then decreased as those cells migrated to other tissues throughout the body. Other patients had a more sluggish maladaptive response: their levels of immune cells hardly wavered from baseline. As you might expect, those with adaptive immune responses recovered from surgery faster.
Most likely it will take decades of new research to gain a much deeper understanding of the biological mechanisms behind such individual differences. For now, though, we can at least be sure that it is okay to feel stressed when you get a shot—in fact, it's a good thing.