Red blood cell evolved a mode of red blood cell development substantially different from the mammalian norm. Overall, mammalian red blood cells are remarkably flexible and deformable so as to squeeze through tiny capillaries, as well as to maximize their apposing surface by assuming a cigar shape, where they efficiently release their oxygen load. Red blood cells in mammals are unique amongst vertebrates as they do not have nuclei when mature. Red blood cells of mammals cells have nuclei during early phases of erythropoiesis, but extrude them during development as they mature; this provides more space for hemoglobin. The red blood cells without
Blood cell in the human body in that they lack a nucleus (although erythroblasts do have a nucleus). The condition of having too few red blood cells is known as anemia, while having too many is polycythemia. White blood cells or "leukocytes", are cells of the immune system involved in defending the body against both infectious disease and foreign materials. They are produced and derived from multipotent cells in the bone marrow known as a hematopoietic stem cells. Leukocytes are found throughout the body, including the blood and lymphatic system. There are a variety of types of white bloods cells that serve