leukocyte n : blood cells that engulf and digest bacteria and fungi; an important part of the body's defense system [syn: leucocyte, white blood cell, white cell, white blood corpuscle, white corpuscle, WBC]

English

Alternative spellings

• leucocyte

Noun

1. In the context of "cytology|immunology": A white blood cell.

Derived terms

• leukocytic
• leukocytoid

Translations

White blood cells, or leukocytes, are cells of the immune system defending the body against both infectious disease and foreign materials. Several different and diverse types of leukocytes exist, but they are all 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.

The number of leukocytes in the blood is often an indicator of disease. There are normally between 4×109 and 11×109 white blood cells in a litre of blood, making up approximately 1% of blood in a healthy adult. In conditions such as leukemia, the number of leukocytes is higher than normal, and in leukopenia, this number is much lower. The physical properties of leukocytes, such as volume, conductivity, and granularity, may change due to activation, the presence of immature cells, or the presence of malignant leukocytes in leukemia.

Etymology

The name "White blood cell" derives from the fact that after centrifugation of a blood sample, the white cells are found in the Buffy coat, a thin layer of nucleated cells between the sedimented red blood cells and the blood plasma, which is typically white in color. The scientific term leukocyte directly reflects this description, derived from Greek leukos - white, and kytos - cell. Blood plasma may sometimes be green if there are large amounts of neutrophils in the sample, due to the heme-containing enzyme myeloperoxidase that they produce.

Types

There are several different types of white blood cells. They all have many things in common, but are all different. One primary technique to classify them is to look for the presence of granules, which allows the differentiation of cells into the categories granulocytes and agranulocytes:

• Granulocytes (polymorphonuclear leucocytes): leukocytes characterised by the presence of differently staining granules in their cytoplasm when viewed under light microscopy. These granules are membrane-bound enzymes which primarily act in the digestion of endocytosed particles. There are three types of granulocytes: neutrophils, basophils, and eosinophils, which are named according to their staining properties.
• Agranulocytes (mononuclear leucocytes): leukocytes characterized by the apparent absence of granules in their cytoplasm. Although the name implies a lack of granules these cells do contain non-specific azurophilic granules, which are lysosomes . The cells include lymphocytes, monocytes, and macrophages.

Overview table

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