(HSC) as an example. HSCs are technically described as: lacking FMS-like tyrosine kinase 3 (Flt3) and lacking the markers specific to discrete lymphoid lineages (Lin), but expressing high levels of Sca1 and c-kit; HSC also express CD44, low levels of Thy1.1 (CD90), but no IL-7Ra or CD27. This is called the (surface) phenotype of an HSC. It can be expressed as a set (Lin2, Sca1high, c-kit high, CD44+, Thy1.1low, CD27 2, and IL-7Ra2). This set is a ‘barcode’ for the HSC, akin to the barcode label attached to your chicken-wing plastic bag for checkout at a supermarket! Scientists use these barcodes to check, categorize and accumulate cells for many purposes often using laboratory methods such as cell flow cytometry. These barcodes partially define the modern meaning of phenotype for leukocytes. Progression of HSC differentiation and lineage commitment is indicated by changes in this phenotype. That is, as the cell changes, the markers will also change and the barcode will change. Typical barcodes for some cell types appearing in this article. Note explaining the barcode parameter details: Flt3 is a cytokine tyrosine kinase receptor thought to be important in early lymphoid development. In addition, Flt3 plays a major role in maintaining B lymphoid progenitors. CD27 plays a role in lymphoid proliferation, differentiation, and apoptosis. The acquisition of CD27 and Flt3 by the HSC coincides with the loss of long-term repopulating potential. At this stage the cells retain both lymphoid and myeloid potential and are referred to as multipotent progenitors. (Medical Immunology, p. 114) Knowledge development regarding lymphopoiesis New questions emerge in immunology continuously as though there were a stem cell for questions. For example, it was thought that the process of lymphopoiesis was a direct, orderly unidirectional sequence. But it is not clear if end-stage lymphocytes come from progenitors that are homogeneous populations or overlapping populations. Nor is it clear whether lineages of lymphocytes develop via a continuum of differentiation with a progressive loss of lineage options or whether abrupt events result in the acquisition of certain properties. Changes in cytoplasm, morphology of the cell nucleus, granules, cell internal biochemistry, signaling molecules and cell surface markers are difficult to correlate with definite stages in lymphopoiesis. The morphological differences do not just correspond to steps in mitosis (somatic cell division), but result from continuous "maturation processes"of the cell nucleus as well as of the cytoplasm and so one must not be too