There are excellent white blood cells and immunity-enhancing antibodies in breast milk, which can easily help build up the immunity of your child, right from when he / she is a baby. With nursing, you can keep him / her safe from:

Urinary Tract Infections

Meningitis

Pneumonia

Diarrhea

Allergies

Ear infections

Sudden Infant Death Syndrome (SIDS)

Research shows that it can also improve brain power and protect babies against colitis, Crohn's disease, insulin-dependent diabetes and some forms of cancer in the future. For the first six-eighteen months of life, moms should breastfeed their babies exclusively, according to The American Academy of Pediatrics.

These should include strawberries, oranges, green beans and carrots. All of these have carotenoids, which contain phytonutrients that can boost the immune system. Phytonutrients can boost the production of white blood cells that combat infections. There is also the antibody called interferon, which blocks out viruses by coating the surfaces of the cell.

According to research, a diet packed in phytonutrients can keep the body safe from heart disease, cancer and other chronic diseases during adulthood.

Less sleep can increase susceptibility for ailments caused by natural killer cells, which damage the immunity. This is true for adults as well as for kids, according to the Center for Holistic Pediatric Education and Research at Children's Hospital, in Boston. Kids in daycare are especially vulnerable to sleep deprivation, given that they find it tougher to go to sleep due to heavy activity.

How much sleep is needed for kids? 10 to 13 hours for preschoolers, 11 to 14 hours for toddlers and up to 16 hours for infants.

You should try to develop the habit of exercising in your kid from early on, which can be a great way to boost his / her immunity. Regular activity increases the natural killer cells in number, and family activities like tennis, basketball, inline skating, hiking and bike riding are great ways to make your child fitter, stronger and better in overall health.

This helps reduce stress on the immune system of your child. You should ensure that kids often wash their hands, preferably with soap. It is important they follow this as well as other methods of ensuring proper hygiene prior to as well as after each meal, after arriving home from daycare, using the bathroom, playing with pets etc. For fast cleanups, keep disposable wipes at home. 

Quit smoking or at least, reduce it as much as possible, to ensure that your kid gets exposed to no smoke or very little smoke. Passive smoking can also be quite harmful for the immunity of children. It causes them to breathe faster, and it reduces the natural detoxification ability of their body. With secondhand smoke, your child can be at more risk of impaired neurological development, reduced intelligence, asthma, ear infections, bronchitis and SIDS. If you can't quit smoking for any other reason, do it for the sake of your kid.

As you probably know, with more sun exposure, there is more Vitamin D for your child. It can be very useful for regulating mood, sleep, appetite and various metabolic processes, and can also give the immunity of your child a natural boost. Thus, it is important to ensure that your child goes out in the sun enough every day.