2 Listen to Beethoven (or Britney)

Listening to music can boost your immunity, but it has to be music you love. "Something that calms one person might rile another," Berk says. "The trick is finding music that soothes your soul." Scientists at McGill University in Montreal found that listening to music that sent "shivers down the spine" or that gave people chills stimulated the same "feel-good" parts of the brain that are activated by food and sex. "Even better than listening to music is making it," says Bittman, who found that people who took part in an amateur group-drumming session had greatly enhanced natural killer-cell activity afterward.

3 Turn down the volume

Noise hurts more than your ears. Any unwanted and intrusive sound can trigger muscle tension, speed heartbeat, constrict blood vessels and cause digestive upsets -- the same response your body has to being startled or stressed. Chronic exposure to noise can lead to even longer-lasting changes in blood pressure, cholesterol levels and immune function. Cornell University research found that women who work in moderately noisy offices produce more of the stress hormone adrenaline and may be more vulnerable to heart disease than women who work in quiet offices. Even worse are unwelcome sounds you perceive as uncontrollable, such as car alarms, barking dogs and P.A. systems. Try to take control over the noise in your environment, even if it means wearing earplugs or asking the restaurant owner or gym manager to turn down the music.

4 Look on the bright side

The immune system takes many of its cues from our thoughts and feelings, so try to keep your outlook upbeat. Years ago, Mayo Clinic researchers found that people who were optimists in their youth tended to live 12 years longer than pessimists. A recent study by Anna L. Marsland, Ph.D., R.N., a psychologist at the University of Pittsburgh Medical Center, found that people who were negative, moody, nervous and easily stressed had a weaker immune response to a hepatitis vaccination than their more positive peers. Negativity is a personality trait that's difficult to change, but if wearing rose-colored glasses can improve your immunity, why not try on a pair?

5 Eat right

Any kind of nutritional deficiency may lead to more frequent and prolonged illnesses. If you starve yourself, your body will think it's under siege and pump out stress hormones. Also, dropping more than 2 pounds per week is hard on the T cells that detect diseased or foreign cells. Best advice: Choose fats carefully (omega-3s are good), get enough protein, eat your fruits and vegetables and drink plenty of fluids. "Dehydration lowers resistance," Berk says. Herbs and supplements that promise to boost immunity do no such thing. In fact, megadoses of some so-called immunity-enhancing nutrients, such as zinc, can backfire, which is why experts recommend avoiding zinc supplements that provide more than 25 milligrams daily. One exception may be echinacea, which seems to lessen cold symptoms' severity.

6 Laugh out loud

While painful emotions like anger and grief can impair health, laughter does the opposite. A real belly laugh increases infection-fighting antibodies and boosts natural killer-cell activity, says Berk, who has shown students funny videos and measured their immune systems' response. "Even anticipating a humorous encounter can enhance immunity," he says. "It happens at the molecular level." Laughter also increases circulation, stimulates digestion, lowers blood pressure and reduces muscle tension.

7 Use your brain

Certain kinds of thinking may boost immunity. University of California, Berkeley, neuroscientist Marian Diamond, Ph.D., found that playing bridge stimulated women's immune systems. Her research is the first to show a connection between the immune system and the part of the brain that handles planning, memory, initiative, judgment and abstract thinking. Says Diamond: "Any mental activity that uses one or a combination of these intellectual functions might benefit immune activity."

8 Move your body

Regular, moderate exercise can boost several aspects of your body's self-defense system. "Physical activity not only strengthens your cardiovascular system," Berk says, "it improves your mood and reduces stress as well." Many studies show that long-term training also elevates natural killer-cell activity. But don't push too hard: If you're already under emotional stress, you might want to avoid exercising beyond your usual level. And if your training is unusually prolonged and intense, your risk for illness and infection goes up (see "How to Stay Healthy," below, for suggestions).

9 Learn how to relax

Stress jacks up your body's production of cortisol and adrenaline, hormones that lower immune response. No wonder you're more likely to come down with a cold or the flu when faced with stressful situations like final exams or relationship problems. Stress-induced anxiety also can inhibit natural killer-cell activity. If practiced regularly, any of the well-known relaxation techniques -- from aerobic exercise and progressive muscle relaxation to meditation, prayer and chanting -- help block release of stress hormones and increase immune function.

10 Douse the night light

Only when it's really dark does your body produce melatonin, a hormone that helps prevent certain diseases. Not sleeping enough, or being exposed to light during the night, decreases melatonin production and boosts estrogen levels, increasing breast-cancer risk. In fact, recent studies have found a heightened risk of breast cancer -- up to 60 percent -- among women who work the graveyard shift, and possibly an even greater increase among women with the brightest bedrooms. Not surprisingly, blind women have an approximate 20-50 percent reduction in breast-cancer risk. Even a dim source like a bedside clock or a night light may switch melatonin production off, so keep your bedroom as dark as possible.

Stress Test: Is your immune system at risk?

Everyone reacts to stress differently. What overwhelms one person may be a motivating challenge to another. "It's not the situation itself that produces negative changes in your immune system; it's how you perceive it," says wellness expert Barry Bittman, M.D. "Some people don't recognize how strongly they're reacting." To learn if your reactions may compromise your immunity, answer these questions, yes or no.

When stressed, do you:
* get tension headaches?
* breathe faster?
* notice that your heart beats faster?
* clench your jaw or grind your teeth?
* suffer gastrointestinal symptoms, such as stomach gurgling or diarrhea?
* experience muscle tension, such as hunched shoulders or a stiff neck?
* feel nervous and jittery?
* perspire? Get chills or clammy hands?
* become upset or angry, irritable, impatient?

Lots of "yes" answers may mean you react to stress in harmful ways. You need to learn coping skills and long-term relaxation techniques. For immediate stress relief, sit quietly, taking slow, deep breaths; relax your shoulders and jaw; visualize a peaceful place; or listen to calming music. Try to change your mind-set too: If you're bugged by a crying baby at the next table, consider how hard it is for infants to communicate their needs; if you're worried about an upcoming exam, remind yourself that you almost always do fine.