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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.
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.
Types There are three major types of white blood cells.
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).
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 become 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.
Monocytes Monocytes share the 'vacuum cleaner' function of neutrophils, but are much longer lived as they have an additional role. Monocytes, and their tissue counterpart macrophages, present pieces of pathogens to T cells so that they may be recognised again and killed, or so that an antibody response may be mounted.
Diseases A type of cancer in which white blood cells multiply out of control is called leukemia.
Other tissue cells - Histiocytes, found in the lymphatic system and other body tissues, but not normally in blood:
- Mast cells
this means there are about 50,000 white blood cells in a drop of blood
External links - A look at the white blood cells of different animals (http://web.vet.cornell.edu/public/popmed/clinpath/CPmodules/heme1/leukocyt.htm)
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