All cellular components of the blood originate from hematopoietic stem cells in the bone marrow. These pluripotent stem cells give rise to two lineages of specialized cells (Fig. 1). The lymphoid lineage produces B- and T-cells; the myeloid lineage gives rise to leukocytes, erythroblasts (the precursors of erythrocytes), and megakaryocytes (which produce platelets) (1).

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