Figure 1. The formation of blood is a hierarchy of development starting with the hematopoietic stem cell (HSC). As the HSC divides, many different types of cells form. Just beyond the HSC, there is a cell that is known as a “hematopoietic progenitor cell” or HPC. Progenitors are not quite the same as stem cells though they share important characteristics. While progenitor cells have an incredible capacity to divide and make other types of cells as they mature, they have only a limited ability to self-renew. The HPC divides and forms more specialized blood cell types that in turn form even more specialized cell types. Ultimately, this generates an array of cells with different functions; our lymphoid blood cells (the B-cells; T-cells; natural killer or NK cells; plasma cells; dendritic cells and others) and our erythroid and myeloid blood cells (the erythrocytes or red blood cells; megakaryocytes or platelet producing cells; granulocytes such as neutrophils, eosinophils, and basophils; and monocytes which make macrophages).