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White blood cell
White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All leukocytes are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, including the blood and lymphatic system.
Five different and diverse types of leukocytes exist. These types are distinguished by their physical and functional characteristics. Monocytes and neutrophils are phagocytic.
The number of leukocytes in the blood is often an indicator of disease, and thus the WBC count is an important subset of the complete blood count. The normal white cell count is usually between 4 and 11 × 109/L. In the US this is usually expressed as 4,000–11,000 white blood cells per microliter of blood.〔(【引用サイトリンク】title=Vital and Health Statistics Series 11, No. 247 (03/2005) )〕 They make up approximately 1% of the total blood volume in a healthy adult. An increase in the number of leukocytes over the upper limits is called leukocytosis, and a decrease below the lower limit is called leukopenia.
==Etymology==
The name "white blood cell" derives from the physical appearance of a blood sample after centrifugation. White cells are found in the ''buff'', 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 word ''leuko-'' meaning "white" and ''kytos'' meaning "hollow vessel", with -cyte translated as "cell" in modern use. The buffy coat may sometimes be green if there are large amounts of neutrophils in the sample, due to the heme-containing enzyme myeloperoxidase that they produce.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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