Young puppies do not have fully developed immune systems so shots of a live virus can affect her body drastically. (caninejournal.com)
In later months the immune system returns to normal and the fetus develops an immune system of its own. (discovermagazine.com)
The list of known breast milk components not present in formula is long and includes hormones, enzymes, growth factors and substances that fight infection and help develop the immune system. (easybabylife.com)