life by cell division and differentiation from cells such as common lymphoid progenitors (CLPs) in mice. The set comprising CLP cells and similar progenitors are themselves descendants of the pluripotential hemopoietic stem cell (pHSC) which is capable of generating all of the cell types of the complete blood cell system. Despite their ability to generate the complete suite of lymphocytes, most progenitors are not true stem cells, however, and must be continually renewed by differentiation from the pHSC stem cell. Many progenitor cells are also referred to as transit cells, sometimes also called transit amplifying cells, the meaning of this term being that the transit cell may found a new sub-lineage but the number of resultant cells is strictly limited (although possibly very large, even trillions yet finite) and the lineage is terminated by cells that die off (by apoptosis) or remain as cells that can no longer divide. Examples of such cells are CFUs (Colony-forming units - referred to as such because of their ability to form colonies in vitro in artificial media) such as CFU-T. In mice, transplantation of a single pHSC cell can reconstitute a sub-lethally irradiated host (i.e. a mouse that has been irradiated so that all leukocytes are killed) with all these lineages of cells, including all types of lymphocytes via CLPs. This has been known for more than 40 years. Lymphopoiesis continues throughout life and so progenitor cells and their parent stem cells must always be present. Overview In the case of mammals such as humans (Homo sapiens), 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. However early in gestation the developing embryo has begun its own lymphopoiesis from the fetal liver. Lymphopoiesis also arises from the yolk sac. This is in contrast to the adult where all lymphocytes originate in the bone marrow. There are four major types of lymphocytes, many sub-types, and hundreds or thousands of lymphocyte cell types that have been identified by scientists. All are generated by normal or abnormal lymphopoiesis except for certain artificial strains created in the laboratory by development from existing strains. Although lymphocytes are usually considered mature (as seen