White blood cell normal when it is part of healthy immune responses, which happen frequently. It is occasionally abnormal, when it is neoplastic or autoimmune in origin. A decrease below the lower limit is called leukopenia. This indicates a weakened immune system. The name "white blood cell" derives from the physical appearance of a blood sample after centrifugation. White cells are found in the "buffy" "coat", a thin, typically white layer of nucleated cells between the sedimented red blood cells and the blood plasma. The scientific term "leukocyte" directly reflects its description. It is derived from the Greek roots "leuk-" meaning "white" and
Hematopoietic stem cell Hematopoietic stem cell Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells. This process is called haematopoiesis.<ref name="Birbrair n/a–n/a"></ref> This process occurs in the red bone marrow, in the core of most bones. In embryonic development, the red bone marrow is derived from the layer of the embryo called the mesoderm. Hematopoiesis is the process by which all mature blood cells are produced. It must balance enormous production needs (the average person produces more than 500 billion blood cells every day) with the need to precisely regulate the number of each blood cell type