Name some natural barriers to infection produced/secreted by different parts of our body. how do infants gain resistance against infection ?

Mucus, saliva, would be natural barriers to infection.

Newborns are much better protected against (or immune to) potential illnesses and diseases than you might otherwise think. This is because during pregnancy, disease-fighting antibodies made in the mother’s immune system are able to make their way across the placenta and into her baby’s body. Fortunately, these antibodies stick around for several months and are able to give newborns an added level of protection from many routine illnesses during this important time when they are not as able to effectively make their own antibodies. 

Ithink it’s around 500 million or something. kinda scary honestly but that’s just my opinion.