Breast milk is the best source to work on babies immunity. In some situations when you cannot feed then you can give formula milk. There are various available in thr market. Recently we have started with Aptamil. Check with your doctor.

Adding prebiotic ingredients to infant formula helps colonize the newborn's gut with a stable population of beneficial bacteria, and probiotics <b>enhance immunity in formula</b>-<b>fed</b> infants

You can give BaalAmrut suvarnaprashan immunity booster , I'm giving this for my daughter https://www.babyorgano.com?ref=339

Give healthy diet to your kids to improve immunity. Make sure that thier intake of prebiotics is sufficient. You can consult with your pediatrician regarding this

Baby's are most vulnerable to infections in early life, especially within first 2 years. One needs to ensure all early/mandatory vaccines are administered to help build immunity. The best way to build immunity is through exclusive breastfeeding. This allows the strengthening of gut through the nutrients passed on to the baby through breast milk. This helps build the immune system. Therefore, with building immunity from an early age, the vulnerability to infection reduces. If you are not able to breastfeed your child, choose a formula which is closest to breast milk. I have heard that my friend uses Aptamil as she could not breastfeed, which apparently is the closest to breast milk. You can try giving that, it has worked wonders for her.