Hematopoiesis, physiological process allowing the creation and renewal of blood cells or hematocytes. Stem cells come from the red bone marrow of the flat and short bones represented here by the pelvic bone. These stem cells give two types of cells: myeloid stem cells which give the differentiated cells following red blood cells or erythrocytes, platelets, white blood cells or leukocytes (polynuclear and monocytes) and lymphoid stem cells which give white blood cells (lymphocytes, which give NK cells (natural killers), T lymphocytes, B lymphocytes, plasma cells).