A baby's immune system is very fragile, and there are a lot of factors which contribute to the strengthening of immune system. The 2 years of a baby's life is extremely crucial for its immune system.The best way to build your baby's immunity is to exclusively breastfeed your baby. Breast milk contains a host of nutrients which are essential for a baby's growth and development. It in fact is the only food that a baby needs especially in the first 6 months of its life.<br> -If you are breast feeding, ensure to keep a check on your diet because what you eat/drink will be passed on to your baby through breast milk.Breast milk plays a crucial role in a child's immunity. However, if you are unable to breastfeed for any reason, you can choose a formula milk which is closest to breast milk. Prebiotics are naturally present in breast milk, and are responsible for enhancing gut health and building immunity. You should choose a formula milk which contains prebiotics.Ensure to take your baby for regular checks with his pediatrician.