During pregnancy, the embryo - or fetus - is normally imbedded in a sterile environment. That is why the formation of(microphages) and (macrophages) occurs only towards the end of the pregnancy.
However, one already finds myeloid stem cells on the umbilical vesicle and in the liver. Phagocyting cells are at birth relatively mature and capable of functioning.
Thego through various stages of development until they reach full maturity. This mainly comprises the formation of functional antigen receptors (T-cells in the thymus) and the ability to form and secrete immunoglobulins (B cells in the bone marrow). A third cell line series of the lymphoid cells are the natural killer cells (NKC). These are large lymphocytes without antigen-specific receptors on their surface. They recognize and kill abnormal cells (tumor and virus-infected cells) and are thus important for the immune defense against intracellular pathogens.
The highly specialized tissue of the primary lymphatic organs is responsible for the maturation of the series of T and B lymphocytes. In this connection one also speaks of a "special microenvironment". When the lymphocytes mature, they get via the blood stream into the secondary (peripheral) lymphatic organs.
(see primary and secondary lymphatic organs)