Keep it upbeat: Optimism may help you fight germs.

RODALE NEWS, EMMAUS, PA—Thinking favorably about your future could actually arm your body to better fight off infection, according to new research. The study, published in the journal Psychological Science, found that certain components of optimism—feeling joyful, attentive, and happy—produced various levels of improved immune functioning in the study participants.

The reasons behind the phenomenon aren't clear. "We know that negative emotions are associated with stress hormones, but positive emotions are likely to be associated with different hormones, ones that we don’t understand as well," says lead study author Suzanne C. Segerstrom, PhD, professor of psychology at the University of Kentucky in Lexington.

THE DETAILS: To understand the study, it's important to understand the researchers' definition of optimism in this particularly study. "Optimism, as I use it, means positive expectations about the future, explains Segerstrom. The expectations can be general beliefs about whether the future holds good or bad things, or they can be specific about how some particular area of one’s life will go.

To analyze optimism's effect on the immune system, researchers enrolled 124 first-year law students in the study. Five times over a six-month period, the students answered questions regarding their levels of optimism and how they felt about law school. Then, they were injected with an antigen that creates a bump on the skin; the stronger the immune response, the bigger the bump. (Immune systems are many-faceted; this test only measures the strength of the part that is responsible for fighting viral infections and some bacterial infections, according to the study authors.)

As the new law students started completing assignments and getting grades back, researchers noted the fluctuations in their optimism levels regarding school. When they were feeling more pessimistic, their immune response was weaker. On the flip side, when they had a more optimistic outlook on law school, they mounted a stronger immune response, which can provide a better defense against some infections. The immune system ups and downs probably don't put generally healthy people like the students at risk, Segerstrom says. But other groups might experience serious consequences. "For example, older adults might be at higher risk because their immune systems are more vulnerable," Segerstrom explains.

WHAT IT MEANS: Other studies have found a connection between people who are optimistic about their health and the time it takes them to heal. And while previous research has looked at immune function in pessimistic people compared to optimistic people, this is the first study to investigate immune-system fluctuations based on optimism levels in the same person. "This relationship between changes in optimism and changes in immunity was the most important finding," says Segerstrom. There may be other factors that differ between people who are more optimistic or less optimistic, like personality differences or behaviors, that affect their health. But, says Segerstrom, "To show that a single person, with the same personality and genes, has different immune function when he or she feels more or less optimistic provides a stronger link between the two."

Read on to find out how to be optimistic.