 Lymphopoiesis continues throughout life and so progenitor cells and their parent stem cells must always be present.In the case of mammals such as humans, lymphopoiesis begins with limited passive provision by the mother of lymphocytes and substantial immunoglobulin G that cross the placenta and enter the fetus to provide some protection against pathogens, and also leukocytes that come from breast milk and enter circulation via the digestive tract.It is often not effective in preventing infections in the newborn.