Your immune system is crucial in maintaining your health—so it’s important to keep it in good order. But how do you do that? People talk about boosting immunity as if it were a task similar to building muscles or reducing blood cholesterol. Hundreds of ads for supplements and other products promise to boost immunity. But keeping your immune system in good shape is a complicated task. "How do I boost immunity?" is really the same question as "What can I do to stay well?"

How does the immune system fight disease?

Because war is a handy metaphor for the human body’s reaction to disease, science writers like to describe the immune system in militaristic terms—the body’s department of defense. But unlike the Armed Forces, the immune system has no headquarters or commander-in-chief. And its operations are usually swifter and more efficient than any army’s could be.

Rather than "making war," your immune system is really more like an immigration service: a highly differentiated cellular bureaucracy that supervises your biological commerce with the outside world, sorts through billions of pieces of information about incoming materials, and takes routine action as required. Only occasionally does it declare an emergency.

The immune system’s basic task is to recognize "self" (the body’s own cells) and "nonself" (an antigen—a virus, fungus, bacterium, or any piece of foreign tissue, as well as some toxins). To deal with nonself or antigens, the system manufactures specialized cells—white blood cells—to recognize infiltrators and eliminate them. We all come into the world with some innate immunity. As we interact with our environment, the immune system becomes more adept at protecting us. This is called acquired immunity.

What are the parts of the system?

Among the primary components of the immune system are a variety of white blood cells. These constitute a communications network that helps organize the immune response.

Most people are surprised to learn that the skin, including the mucous membranes, is among the most vital components of immunity. The skin not only forms a wall against intruders, but actually alerts the white blood cells if the wall is breached by invading organisms (through a wound, for instance). The protection afforded by the intact skin is why it’s nearly impossible to catch a disease from a toilet seat, for example.

Most infectious agents get inside the body when we inhale them or swallow them; a few can enter through the genitals. They make their way into the blood and move rapidly through the body. The immune system has its own circulatory system called lymphatic vessels, which allow white blood cells to catch intruders. Other important parts of the immune system include the tonsils and adenoids, thymus, spleen, lymph nodes, appendix, certain areas of the small intestine, and bone marrow.

What do the white cells do?

Many mature white blood cells are highly specialized. The so-called T lymphocytes (T stands for thymus-derived) have various functions, among them switching on various aspects of the immune response, and then (equally important) switching them off. Another lymphocyte, the B cell, manufactures antibodies. A larger kind of white cell, the scavenger called the phagocyte (most notably the macrophage), eats up all sorts of debris in tissue and the bloodstream, and alerts certain T cells to the presence of antigens.

In addition, there are killer, suppressor, and helper T cells. Killer T cells, stimulated by helper T cells, zero in on cells infected by antigens, or turn against the body’s own cells when, as in the case of cancer, they begin to proliferate abnormally. Another class of lymphocyte killer cell is called "natural" because, unlike T and B cells, it doesn’t need to recognize a specific antigen. Most healthy cells are of no interest to natural killer cells, but cancer cells and cells invaded by viruses may be vulnerable to their search-and-destroy missions.

What makes a person immune to various diseases?

Thanks to the lymphocytes, the immune system possesses a memory, or a sense of history. The lymphocytes manufacture antibodies (proteins circulating in the blood) that attack intruders. Once you have produced antibodies to a certain microbe—a specific flu virus, for example—that particular virus cannot make you sick again, because you have cells that immediately recognize it and produce the antibodies that destroy it. The immunity may last for years, sometimes for life. This is "acquired immunity."

Science has also developed vaccines. It all began in the late eighteenth century, when the English physician Edward Jenner observed that people who caught a mild disease called cowpox never got smallpox, which is related to it. Using a boy who had not had either disease, Jenner tried inoculation: he scraped the child’s skin and applied secretions from cowpox sores, and the boy got cowpox. When Jenner later inoculated him with smallpox matter, the boy did not develop smallpox. (Such human experimentation would land Dr. Jenner in court today.)

Creating immunity by injecting healthy people with dead or altered disease-causing microbes has prevented millions of deaths from measles, polio, diphtheria, flu, smallpox, tetanus, yellow fever, and many other diseases. Vaccines truly are immune-system boosters.

Does loss of sleep depress immunity?

It can. But losing sleep for a few nights won’t necessarily make you ill. Many things boost or depress immunity temporarily. The number of immune cells rises and falls naturally in healthy people.

What foods boost immunity?

An adequate diet helps maintain immunity and keeps you healthy. The immune system needs such nutrients as protein, fatty acids, vitamins, and minerals. Severely malnourished people are particularly vulnerable to immune dysfunction, and they get sick more easily than other people and stay sick longer. What most people want to know, though, is whether one specific food or kind of food will boost immunity in otherwise healthy people on an adequate diet. The answer is generally no.

What supplements boost it?

So far there’s no reason to believe that supplements will boost immunity in healthy people, except in the malnourished and many of the elderly. Though severe malnutrition is rare in the U.S., some groups, particularly the elderly, may be deficient in such nutrients as vitamin C, certain B vitamins, and zinc. Studies suggest that raising nutrient intakes to adequate levels can enhance immunity, and there is some evidence that elderly people stay healthier if they take a multivitamin/mineral pill. In contrast, other research suggests that megadoses of certain nutrients can significantly suppress some immune responses.

Consider zinc, for instance, found in meat and grains, and often promoted as an ideal immune-system booster when taken as a supplement. While some studies show that zinc supplements can boost immunity and promote wound-healing in the elderly, high intakes can actually suppress the immune response.

A diet low in beta carotene can depress immunity, but it’s not clear that beta carotene supplements can correct the situation, or what levels of supplementation would be helpful. Among the agents that have been shown to stimulate immunity in experiments are bacteria such as those in yogurt, but it’s far from certain that consuming yogurt (with or without live cultures) will promote resistance to disease.

What about vitamin C?

This vitamin is necessary to good health and no doubt to immune function. But numerous studies have shown that vitamin C supplements have minimal or no effect on the immune response, unless you are deficient in C.

Does exercise boost immunity?

Some research shows that sedentary people don’t have as vigorous an immune system as those who exercise. Moderate exercise (for example, a moderate walking program undertaken by previously sedentary people) seems to improve immune function. But there is also evidence that overdoing exercise may depress the immune system: high-intensity or prolonged endurance exercise steps up the output of two so-called stress hormones, adrenaline and cortisol, both of which can depress various components of the immune system.

Olympians and other highly trained athletes often report that after intense competition and training they are more susceptible to colds. Yet such news should not deter athletes from competing or exercisers from exercising.

The health benefits of exercise are clear. Regular aerobic exercise is good for the heart. Weight-bearing exercise builds bone and muscle. The idea that your immune cells might not show a response to your exercise program should not deter you from exercising or from beginning an exercise program if you are sedentary.

Can emotions affect the immune system?

States of mind surely affect health, and extreme emotional stress may damage immunity and bring on illness. But research into the link between mind and immunity is in its early stages and has produced very little solid evidence so far—and not much advice about how to protect the immune system from the ill effects of emotional stress. An experiment may show that extreme grief depresses human T cells, for instance, but we don’t know if the rest of the system is harmed, or whether the fluctuation means much.

Still, reports of increased illness and even death among the recently bereaved are common. Cancer patients with a "fighting spirit" seem to live longer than those who are despondent, but this may or may not prove something about immune function. Good social support is thought to improve immunity in people under stress.

Immune cells and nerve cells do interact. For example, when fighting an infection, immune cells are able to stimulate the brain to transmit the impulses that produce fever. Receptors for many of the chemicals released during stress, such as epinephrine and norepinephrine, have been observed on the surface of lymphocytes found near nerve terminals in the lymph nodes and spleen. This suggests that what goes on in the brain can interact with the immune system to suppress or, conversely, enhance it.

What does smoking do to immunity?

Part of the reason smokers are at risk for lung cancer and respiratory diseases may be that smoking suppresses immune cells. When smokers quit, immune activity begins to improve within 30 days.

When and why does the immune system malfunction?

The immune system has so many built-in fail-safes that, in theory at least, we should rarely fall ill. But, in fact, we do. Harmful agents such as HIV can baffle our defenses. The system can simply be overwhelmed by the number and toxicity of viruses, bacteria, or other foreign cells and toxins.

Though the immune system defends us against cancer, it is subject to cancer. Leukemia is a cancer of the white blood cells; multiple myeloma affects certain lymphocytes that produce antibodies. Cancers of the lymph system include lymphoma and Hodgkin’s disease. Some of these cancers can now be successfully treated.

Sometimes the gatekeepers of the system go crazy, mistaking a basically inoffensive intruder such as pollen, dust, or a bit of bee venom for an enemy and causing the body to go into the red alert known as an allergic reaction. In addition, the immune system can mistake the body’s own cells and tissues for "nonself" and attack them, as in auto-immune diseases like rheumatoid arthritis and systemic lupus.

The immune system will also reject and kill potentially lifesaving organ and tissue transplants, unless some way can be found to circumvent the reaction. Though in theory a pregnant woman’s immune system should attack the fetus—which is nonself—it doesn’t. This is because the fetus itself produces a substance that shields it from the maternal defense system.

So how can I nurture my immune system?

Perhaps the most direct action you can take is to consume a varied, balanced diet of vegetables, fruits, whole and fortified grains, and dairy products, with small amounts of fish and meat if you wish. A basic daily multivitamin/mineral supplement is usually a good idea for older people. Beware of any supplement, however, that promises to boost immunity: protein supplements, enzyme supplements, and the whole range of specific vitamins and minerals, antioxidants, and nostrums that claim to boost immunity don’t strengthen it.

Regular moderate exercise is associated with good health and longevity and will benefit your cardiovascular system, whether it boosts immunity or not. Getting adequate sleep is also helpful. And, of course, don’t smoke.