White blood cell

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White blood cells (also called leukocytes or immune cells) are a component of blood. They help to defend the body against infectious disease and foreign materials as part of the immune system. There are normally between 4x10⁹ and 11x10⁹ white blood cells in a litre of healthy adult blood - about 7 000 to 25 000 white blood cells per drop. In conditions such as leukemia this may rise to as many as 50 000 white blood cells in a single drop of blood. As well as in the blood, white cells are also found in large numbers in the lymphatic system, the spleen, and in other body tissues.

1 Nomenclature
2 Types
3 Diseases
4 Other tissue cells
5 See also
6 External links

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Nomenclature

The name "white cells" derives the from the fact that after centrifugation of a blood sample, the white cells are found in the Buffy coat, a small fraction between the hematocrit and the blood plasma, which is white in color (or sometimes green, if there are large amounts of neutrophils in the sample, which are high in green myeloperoxidase).

Any of various blood cells that have a nucleus and cytoplasm, separate into a thin white layer when whole blood is centrifuged, and help protect the body from infection and disease. White blood cells include neutrophils, eosinophils, basophils, lymphocytes, and monocytes. Also called leukocyte, white cell, white corpuscle.

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Types

There are three major types of white blood cells.

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Granulocytes

Granulocytes are a category of white blood cells, characterised by the fact that all types have differently staining granules in their cytoplasm on light microscopy. There are three types of granulocytes: neutrophils, basophils, and eosinophils (named according to their staining properties).

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Lymphocytes

Lymphocytes are much more common in the lymphatic system, and include the so-called "killer T-cells". The blood has three types of lymphocytes: B cells, T cells and natural killer cells. B cells make antibodies that bind to pathogens to enable their destruction. CD4+ (helper) T cells co-ordinate the immune response (they are what becomes defective in an HIV infection). CD8+ (cytotoxic) T cells and natural killer cells are able to kill cells of the body that are infected by a virus.

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Monocytes

Monocytes share the 'vacuum cleaner' function of neutrophils, but are much longer lived as they have an additional role. They present pieces of pathogens to T cells so that the pathogens may be recognised again and killed, or so that an antibody response may be mounted. Monocytes are also known as macrophages after they leave the bloodstream and enter tissue.

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Diseases

Leukopenia is a disease symptom defined as a lower than normal number of white blood cells in the blood.
Leukocytosis refers to an increase in the number of white blood cells in the blood.
Leukemia and lymphoma are two types of cancer in which white blood cells multiply out of control.

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