How to improve immune system?

Eat Healthy, stay healthy. Yes, eating well is the key to keeping your immune system in perfect working order. Eat lots of vegetables and fruits to get in your required dose of vitamins, minerals and anti-oxidants. They give your body the power to fight off nasty infections. Regular, moderate exercise is also a good way to improve your immune system. Go for a brisk 30-minute walk at least 4 days a week. It keeps your blood circulation moving and helps get rid of fatigue that lowers the resistance level of your body. Regular intake of herbal teas and lemon juice keeps your immune system ticking. Sleep well so that your body has the strength to face up to viral and bacterial attacks.

The following foods help in increasing the immunity of our body.
Vitamin C
Sources: bell pepper, broccoli, Brussels sprouts, cauliflower, fruit juices, lemon juice, mustard greens, oranges, papaya, strawberries. Foods with vitamin C increase the production of infection-fighting white blood cells and increase levels of interferon, the antibody that coats cell surfaces preventing the entry of viruses.
Vitamin E Sources: almonds, broccoli, chard, mustard greens, olives, papaya, sunflower seeds, turnip greens. Vitamin E stimulates the production of natural "killer" cells (cells that seek out and destroy germs and cancer cells). Vitamin E enhances the production of B-cells, the immune cells that produce antibodies that destroy bacteria. Vitamin E may also reverse some of the decline in immune response commonly seen in aging.
Carotenoids-The carotenoid vegetables are those which are yellow, orange, or red in color and contain carotenes, such as beta-carotene. Beta carotene increases the number of infection-fighting cells. Beta carotene is only one member of a large family. Researchers believe that it is not just beta carotene that produces all these good effects, but all the carotenoids working together. This is why getting carotenoids in food may be more cancer-protective than taking beta carotene supplements. You should eat at least five servings of fruit and vegetables each day.
Vitamin B12
Sources: beef, halibut, lamb, milk, salmon, scallops, shrimp, snapper.
Vitamin B12 is central to immune processes because, without adequate B12, white blood cells can't mature and multiply. Folic acid also plays a key role in immune system development and maintenance.
Niacin (B3)
Sources: asparagus, chicken breast, cranberries, halibut, mushrooms, pork, salmon, tomato, tuna

Pyridoxine (B6)
Sources: bell peppers, cauliflower, cranberries, mustard greens, tuna, turnips

Folate
Sources: asparagus, beets, broccoli, calf's liver, lentils, parsley, romaine lettuce, spinach, turnip greens

Zinc
Sources: oysters, mushrooms, meat, legumes

This valuable mineral increases the production and effectiveness of white blood cells that fight infection. Zinc also increases killer cells that fight against cancer and it helps white cells release more antibodies. Zinc also increases the number of infection-fighting T-cells, especially in elderly people, who are often deficient in zinc and whose immune system often weakens with age. The anti-infection hype around zinc is controversial. While some studies claim that zinc supplements in the form of lozenges can lower the incidence and severity of infections, other studies have failed to show this correlation. A word of caution: too much zinc (more than 75 milligrams a day) in the form of supplements can inhibit immune function.

Chromium
Sources: brewer's yeast, oysters, liver, onions, whole grains, bran cereals, tomatoes, potatoes

Many people do not get enough chromium in their diet due to food processing methods that remove the naturally occurring chromium in commonly consumed foods. Recent research in animal models shows that chromium can enhance the ability of white blood cells to respond to infection.

Selenium
Sources: Brazil nuts, brown rice, cottage cheese, chicken (white meat), egg yolks, garlic, halibut, lobster, mushrooms, pork, salmon, shrimp, snapper, sunflower seeds, tuna, whole grains

This mineral increases natural killer cells and mobilizes cancer-fighting cells.

Your diet should contain above mentioned nutrients to boost your immune system.