There is no magic potion. Herbal remedies such as echinacea and ginseng may help, and zinc sometimes seems to shorten the duration of cold symptoms, but the jury is still out.
"All of these various remedies you can buy, frankly, most of them are untested," said David L. Woodland, an immunologist at the Trudeau Institute, a nonprofit research center in Saranac Lake, N.Y.
Woodland says the real secret to staying healthy is staying healthy.
"The most important thing you can do is to actually stay very healthy," he said. "The immune system is directly related to your general state of health."
One well-established drag on the immune system is stress. Chemicals released by the body during periods of strain suppress the immune system, Woodland said. This fight-or-flight response to dangerous or urgent situations was probably fleeting for our human ancestors, he said.
"The problem in our modern world," he said, "is that we're under stress for long periods of time."
Therefore, any kind of stress-relieving activity--from various forms of meditation to exercise--is an investment in a healthier immune system.
Woodland notes that aging takes a toll on the immune system. People do tend to get fewer colds as they age because their immune systems have developed defenses against many kinds of cold viruses.
However, he said, at about the age of 70, "the immune system decline outweighs the benefits you've been accruing," and the individual becomes more prone to illness. That doesn't mean, however, that an older immune system is a sitting duck.
Dr. Janet McElhaney, a geriatrician at the University of Connecticut Health Center in Farmington, said that a well-balanced diet is a key to a stronger immune system for people of all ages.
For the elderly, she said, the concern is whether they're getting all of the nutrients they need daily. Of particular concern, she said, is protein, because it has a direct effect on the blood cells in the body that fight infection. Also, a multivitamin or two can be helpful in older people.
Sleep is another staple of robust immunity. McElhaney said older bodies tend to cool down too early in the morning and that this can awaken an individual at 3 a.m. or so.
"We recommend that older people not turn down their thermostats at night, or [to] use electric blankets or take a hot bath at night," she said.
For kids, the immune-building issues are the same, although specific problems are different.
"Many kids who go to school don't eat breakfast, and that's not a good thing," said Dr. Michael Frogel, chief of the division of general pediatrics at Schneider's Children's Hospital of Long Island Jewish Medical Center in New Hyde Park, N.Y.
He said good nutrition is a must for kids year-round, and that kids, like adults, need plenty of sleep and regular exercise. Frogel said that watching more than two hours of TV a day has been shown to promote obesity in kids: "Even if you're watching TV, get up and move around."
Some studies suggest that dropping a few pounds has immune-building health benefits. A team at Tufts University's Friedman School of Nutrition Science and Policy found that when a group of adults with high cholesterol lost weight on a low-fat diet, most also showed evidence of improved immune function.
A moderate balanced diet, exercise, sleep and stress relief--it sounds like a recipe for overall health, and that's just what it is, said Anthony Vella, an assistant professor of immunology at the University of Connecticut.
Vella, who is working on a new generation of more effective vaccines, said people who engage in a healthier lifestyle should know that these things don't directly manipulate the immune system the way vaccines do. Nonetheless, such an effort is worth it.
"General health is good not just for the immune system, but for lots of things," he said.