Feeling optimistic may actually help you maintain your health, say psychologists writing in Psychological Science.

The authors claim they have found "the first evidence that changes in optimistic expectancies are accompanied by changes in immunity, as well as the first evidence for a mechanism by which this effect occurs."

The conclusion is based on a study of how law students' expectations about their schooling affected their immune responses.

Other studies have found that people who are optimistic about their health tend to do better. For example, people who are optimistic about heart transplant surgery recover better from that grueling operation. But it's not clear how optimism affects your health — or whether pessimism makes you less healthy.

For the new study, 124 first-year law students were observed five times over six months. Each time, they answered questions about how optimistic they felt about law school. Then they were injected with material that should summon an immune response and two days later, they came back to have the injection site measured. A larger bump in the skin means a stronger immune response. 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.

The students' general outlook on life — whether they had an optimistic disposition — didn't account for the differences in immune responses between students. But as each student's expectations about law school waxed and waned, their immune response followed along. At more optimistic times, they'd have bigger immune responses; at a more pessimistic time, a more sluggish immune response. So, being optimistic about success in a specific, important domain may promote better immunity against some infections.

Of course, the law students often have good reason to be optimistic or pessimistic; by a few months into the first semester, they've gotten some grades back and started to figure out if they're good or bad at law school. "I don't think that I would advise people that they should revise their expectations to be unrealistic," says Suzanne Segerstrom, Psychologist at the University of Kentucky. "But if people have slightly more positive views of the future than is actually true, that's adaptive."

Citation: Suzanne C. Segerstrom, Sandra E. Sephton, 'Optimistic Expectancies and Cell-Mediated Immunity The Role of Positive Affect', Psychological Science, February 2010; doi: 10.1177/0956797610362061