Stem cells in your baby's bone marrow produce red and white blood cells, and platelets-the cells that clump together to form a blood clot. Earlier in pregnancy all of these were produced in the yolk sac (see You are 5 Weeks and 2 Days), then the liver and spleen. Now every bone inside your baby contains red bone marrow capable of producing blood cells. Red blood cells do not last forever and after about 80 days are removed from the baby's circulation. This turnover is higher than your own, where a red blood cell will last for 120 days.
Bilirubin is a breakdown product from red blood cells. It's produced in the liver and removed from your baby's circulation by the placenta. Because your baby's liver takes a few days to efficiently process bilirubin, high levels may lead to jaundice at birth . If jaundice does develop after birth then light phototherapy treatment is capable of breaking down bilirubin into a form that can be more easily excreted in your baby's urine.
As a newborn, your baby will be protected against infection by white blood cells and by antibodies from breast milk, especially the colostrum . Because of this, breast-fed babies are at a lower risk of conditions such as asthma, cow's milk intolerance, and food allergies.