What is the immune system?
The immune system is a defense system that helps our body fight with various infections and toxic agents. Immune system’s role is to prevent these organisms from entering our body or when they do, to prevent them from spreading to other parts of our body. A healthy immune system helps us to feel better, to look better and to use our energy more efficiently. It protects us against infections, cancer and environmental threats. A well-functioning immune system also supports the healing process of our body after burns or surgeries. The immune system gradually develops throughout the life of the baby and learns to fight with microorganisms and in time. Through time, an immunological memory is created. Note that the weakest period of the immune system is between age of 1 and 3 and strongest period is in our adult life.
The immune system varies from childhood to adulthood. Each person is born with a strong but not a fully developed immune system. The only antibody that is transferred to the fetus in mother's womb via placenta is IgG. IgA level in breast milk is remarkably high and it directly acts an antibody against viruses. For this reason babies that are not breast-fed have higher risk of infection and diseases.
Nutrition is very important in terms of boosting our immune system by fighting with free radicals. If we are planning to eat in order to defend our body, cholesterol and fat intake along with protein and fiber intake are as important as variety of foods we eat. At this point we should avoid fat meat and meat with too much sauce and excessive sugar consumption. Sugars reduce the ability of white blood cells to destroy unfriendly bacteria. Also to stimulate immune response, we need to drink plenty of water.
One harsh reality that all moms face is that,  as your child begins to socialize and meet more people, he will start to get sick. This is normal and expected. Keep in mind that he may need a little support to keep his strength. A strong immune system is built on these:
Vitamin C:

Your little gourmet needs Vitamin C and other anti-oxidants every day. Fresh vegetables and fruits are good sources of Vitamin C. The vegetables and fruits with bright shiny colors are rich in antioxidants.
Vegetables rich in Vitamin C: Tomato, onion, garlic, cucumber, sun root, leek, green cabbage, red cabbage, cauliflower, purslane, broccoli, zucchini, sweet peas, bell pepper, artichoke, beans, parsley

Fruits rich in Vitamin C: Pomegranate, blueberry, rose hip, lemon, orange, strawberry, grapefruit, kiwi, raspberry

Vitamin A, C and E, anti-oxidants like beta carotene, zinc and selenium are crucial for a strong immune system.  Your little gourmets need antioxidants to protect the body against diseases and to live a healthy life. Carrot, zucchini, winter squash, leek leaves, broccoli, sweet potato, tomato, strawberry, blueberry, cantaloupe, peach, apricot, citrus fruits, pepper, dried fruits, seeds, whole wheat, kiwi, egg, fish and sea foods will help you a lot and will help provide the strength your child needs.
Vitamin E and selenium helps to boost the immune system. Vitamin E lowers risk of heart attack both for men and women and protects us against many types of cancer. Sunflower seed oil, olive oil, hazelnut, almond, soy bean, walnut and nut types are good sources of Vitamin E.
Orange, red and green color vegetables and fruits are rich in beta carotene that enhances immune cell functions. By including in your diet the foods that contain this vitamin, you both boost your immune system and protect your body against cancer. The body converts beta-carotene into Vitamin A, an essential nutrient.
Vitamin A sources are carrot, spinach, zucchini, tomato, liver, broccoli, lettuce, apricot and cantaloupe. Vitamin A naturally increases your white blood cell count and helps the body fight with cancer tumors.
Proteins and essential fatty acids: 

Your baby needs adequate white blood cells to fight with infections. And she will obtain these from high quality forms of protein.
Adequate protein protects the body against diseases. The crucial point here is this: You should be careful with meat, chicken, fish as these foods are high in saturated fat. So, why not go for a lovely legume? Eat legume at least twice a week. They are high in protein and B group vitamins, Vitamin E and minerals. So, they may be loyal friends to spend some quality time with. Egg is another source of high quality protein and children should eat at least one egg a day during childhood and adolescence.
Besides these, to enhance immunity of your baby you need essential fatty acids. By fatty acids we mean having oily fish, dried fruits, sunflower seeds and other seeds in your lovely and yummy meals. 

Probiotics are healthy bacteria that will help optimize your intestinal flora and boost the immune system. They play a critical role in preventing and treating diarrhea and food allergies. They will help your digestive system run smoothly and support vitamin synthesis. Sounds like superman, right? Almost…The most important probiotic is yoghurt. Yoghurt is produced by bacterial fermentation of lactose in milk and acts like a probiotic thanks to its friendly bacteria content. Thus it boosts immune system of children and adult as well.
Probiotic bacteria are available in high levels in gut flora of the babies fed with breast milk. That helps breast milk to prevent not only disease causing organisms but also the immune system diseases like food allergies. Your baby needs a strong digestive system in order to have a strong immune system. Yoghurt and banana are good sources of probiotic.

These have has various functions in the body. The most important mineral supporting immune system and cause weakening of body resistance in case of deficiency is zinc. Zinc available in whole-wheat flour, hazelnut, almond, cheese, seafood, egg, meat and chicken protects us against infections.
Vitamin B6

It plays a role in regulating the immune system. Meat, fish, whole wheat bread, green leafy vegetables are good sources of Vitamin B6. With an adequate and balanced diet, daily Vitamin B6 need is met.
Folic Acid

Foods of animal origin that are rich in proteins are also good sources of B12. Meat, fish, legume, cheese, egg are main sources of folic acid. Vitamin B12 boosts the immune system
Drinking liquids/water

The right choice will be to meet your fluid need with water, buttermilk drink and fresh-squeezed fruit juice.
What are the other foods that enhance resistance of the body?
Garlic is one…It is also said to help common cold... The reason lies in its it's allicin content. Allicin has antibacterial and antiviral functions. It is also responsible for the strong odor of the garlic. Garlic also has anti-inflammatory properties that help reduce the risk of asthma in adult life of the baby.
Along with garlic, radish, quince and pineapple are some of the other foods that will enhance your little gourmet’s body resistance.