rigid about morphological distinctions between certain cell stages. Models and updates on the lymphopoiesis family tree Until recently the model of the CMP generating all myeloid cell and the CLP generating all lymphoid cells was considered necessary and sufficient to explain the known facts observed in the generation of WBCs, and it is still found in most basic textbooks. However beginning around 2000 and gaining momentum after 2005 in both studies in humans and mice, new complexities were noted and published in papers. These studies are important now mainly to immunology researchers but are likely to eventually lead to changes in medical treatments. The changes were sparked by observations that lymphopoiesis did not always break into two lineages at the level of the CLP. Worse, some macrophages (long considered a myeloid lineage) could be generated by lymphoid lineage progenitors. In essence focus has been shifted away from the CLP to the MLP (lymphoid-specified progenitors), which are clearly lymphoid progenitors yet retain some myeloid potential, particularly the ability in both humans and mice to make macrophages - one of the most versatile of immune cell defenders - and also many dendritic cells, the best 'watchdogs' of antigen invaders. However, whatever the details may turn out to be, the process of lymphopoiesis always seems to relentlessly give rise to progeny with special attributes and abilities - "superpowers" so to speak - but with progressively more restricted lymphoid developmental potential. Stages of development The old model: lymphoid vs myeloid This model of lymphopoiesis had the virtue of relative simplicity, agreement with nomenclature and terminology, and is still essentially valid for the laboratory mouse. pHSC pluripotent, self-renewing, hematopoietic stem cells which give rise to MPP multipotent progenitors, which give rise to ELP (or PRO) Prolymphocytes, early lymphoid progenitors, and finally to the CLP Common lymphoid progenitor, a cell type fully committed to the lymphoid lineage. pHSC, MPP and ELP cells are not fully committed to the lymphoid lineage because if one is removed to a different location it may differentiate into non-lymphoid progeny. However CLP are committed to the lymphoid lineage. The CLP is the transit cell responsible for these (generally parallel) stages of development, below: NK cells Dendritic cells (lymphoid lineage; DC2 ) Progenitor B cells Pro-B cells => Early Pro (or pre-pre)-B cells => Late Pro (or pre-pre)-B cells Large Pre-B cells => Small Pre-B cells Immature B cells B Cells =>