Double your immune power.

You can't see it, but chances are your immune system is at work right now, wiping out armies of bacteria and viruses that can cause all sorts of ailments. Every day your defense system fights off disease and infection. Like an invisible shield against invisible predators, a healthy immune system enables you to:

* escape "nuisance" ailments such as sore throats, colds, and the flu

* travel to foreign lands without getting sick (well, usually)

* heal quickly after a cut, scrape, or burn

* minimize damage from pesticides or other toxins that accidentally make their way into the air or water supply

* resist serious diseases, such as heart disease and cancer.

Revitalize Your Defenses. About three out of four people find that as they age they're slightly more susceptible to "whatever's going around." Apparently that's unnecessary. By improving nutrition and taking good care of your body's natural defenses, your immune system can remain healthy even as you grow older. Here are some ways to bolster your defense system.

Eat well. One of the tastiest and easiest ways to improve your immune system is to load up with the right groceries. "The best advice is to eat the healthiest diet you can--one emphasizing fruits, vegetables, and grains," says William Pryor, Ph.D., professor of chemistry and biochemistry and director of the Biodynamics Institute at Louisiana State University in Baton Rouge.

These foods contain nutrients--particularly beta-carotene and vitamins C and E--that have been shown to help fight diseases. These powerful nutrients are known as antioxidants and help battle the effects of free radicals, unstable molecules in your body. "We used to think that the effects of antioxidants were small," says Pryor. "Now we're finding that the effects are quite potent."

Also, vitamin C stimulates production of interferon, a substance that prevents viruses from taking hold and helps keep white blood cells healthy. Vitamin E stimulates the production of "killer" cells that seek out and destroy viruses, bacteria, and cancer cells, says Ronald Ross Watson, Ph.D., an immunologist at the University of Arizona School of Medicine in Tucson. Beta-carotene stimulates macrophage cells--white blood cells that patrol your bloodstream and can produce chemicals that can kill cancer cells.

Consider a supplement. Even small additional amounts of certain nutrients can boost immune power, according to a study involving 96 people aged 66 and older. One group received a supplement with moderate amounts of 18 nutrients, while the others received a pill similar in appearance but with only calcium and magnesium. The folks who got the 18 nutrients had fewer infections and only 23 sick days that year, while the other group had 48 sick days. Blood tests showed that the supplemented group had stronger immune systems, with more immune cells and a stronger immune response to viruses. Roughly one third of the group was low in some nutrients when the study started, which isn't uncommon in older folks. "For most people, you'd like to bolster the diet naturally with healthy foods like fruits and vegetables," says Ranjit Kumar Chandra, M.D., university research professor at Memorial University of Newfoundland and director of the World Health Organization Center for Nutritional Immunology. But if that's difficult or impossible, a balanced supplement would be the answer, he says.

The supplements in this study contained approximately the recommended dietary allowance (RDA) levels of iron, zinc, copper, selenium, iodine, calcium, magnesium, vitamins A, C, and D and the B vitamins, plus extra beta-carotene and vitamin E. If you choose to take a supplement, reach for a balanced vitamin and mineral supplement that doesn't exceed 100 percent of the RDA, say experts.

Learn to manage stress. The evidence is in: stress does make you more susceptible to colds. In a landmark study researchers gave cold viruses in nasal drops to 400 volunteers, while 26 people received drops without the cold virus. It turned out that the people who were identified as the most stressed were twice as likely to get colds as the others.

Stress may stimulate hormones that help suppress the function of immune systems," says Redford B. Williams, M.D., professor of psychiatry and director of the Behavioral Medicine Research Center at Duke University Medical Center in Durham, North Carolina. "Helping people cope with stress may not just reduce infections, but research indicated it may also lower risks for heart disease."

Get a move on. Moderate exercise stimulates the activity of immune cells and makes your whole immune system work better, concludes a report in Sports Science Review. Studies have shown that moderate exercise improves the ability of macrophage cells to wipe out invaders, including cancer cells. Also, moderate exercise is associated with fewer occurrences of colon, prostate, and breast cancers. ("Moderate" means at least 30 minutes of aerobic exercise, such as walking or bicycling, three to five times a week.)

Catch plenty of Z's. During sleep, your body slows down certain activities and even shuts down others so it can devote itself to healing. When you're feeling ill or run-down, a good night's sleep will help rebuild your defenses.

Hide out from free radicals. Free radicals are the unstable molecules that form the basis of many major diseases. Some free radicals result naturally from reactions within your body, but still more come from inhaling smoke, eating a fatty diet and being exposed to the ultraviolet rays of the sun. Here are three easy steps to reduce exposure to free radicals:

* If you smoke, quit. (If you don't smoke, avoid breathing in smoke from other people.)

* Reduce fat in your diet.

* Protect your skin from the sun with clothing and sunscreens--and by avoiding peak sunlight hours (usually 11:00 a.m. to 2:00 p.m.) whenever possible.

Get a flu vaccination. Research suggests that flu shots not only prevent viral flu infections but, if taken yearly, can also power up the immune system. When a group of adults in their 70s were given a flu vaccine, their declining immune systems were so bolstered that they matched those of a group of young adults. The vaccinated people were resistant against not only the particular flu strains in the vaccine but other strains as well, says Janet E. McElhaney, M.D., assistant professor of medicine at the University of Alberta in Edmonton.

Opt for cultured yogurt. Yogurt with live cultures may also help boost the immune system. In one study, after 68 people ate two cups of live-culture yogurt daily for three months, they produced more gamma interferon--a substance produced by disease-fighting white blood cells--than people who ate heat-treated yogurt with no active culture, reports the International Journal of Immunotherapy. Results persisted two months after people quit eating the yogurt, says Georges Halpern, M.D., adjunct professor of medicine in the department of internal medicine at the University of California at Davis. He thinks the bacteria in yogurt stimulate parts of the immune system in the intestinal tract.

Gauging Your Immunity. There are numerous tests to discover specific strengths and weaknesses of the immune system, but the simplest and most inexpensive is the complete blood count (CBC). This test reveals the number of white blood cells you have, which alerts your doctor to any imbalances in your immune system.

Other tests can measure the effectiveness of your T-cells--a type of white blood cell that's important in the immune system--and detect the presence of individual immune cells and immune chemicals. Doctors can also test for deficiences of specific nutrients.