White blood cell

From Wikipedia, the free encyclopedia

White blood cells or leukocytes are cells which form a component of the blood. They are produced in the bone marrow at a rate of about 1 billion per day^{[citation needed]} and help to defend the body against infectious disease and foreign materials as part of the immune system.

A scanning electron microscope (SEM) image of normal circulating human blood. One can see red blood cells, several knobby white blood cells including lymphocytes, a monocyte, a neutrophil, and many small disc-shaped platelets.

There are normally between 4×10⁹ and 1.1×10¹⁰ white blood cells in a litre of healthy adult blood. In conditions such as leukemia this may rise to as many as 50,000 white blood cells in a single drop of blood (only 1% of blood on average is a white blood cell). 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.

The physical properties of leukocytes (Volume, Conductivity, Light Scatter, etc.) may change, due to activation, due to the presence of immature cells as immature netrophils, or due to the presence of malignant leukocytes in leukemia.

[edit] Nomenclature

The name "white cells" 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. (Plasma may sometimes be green, if there are large amounts of neutrophils in the sample, which are high in green. This green colour is due to neutrophils having myeloperoxidase (MPO).)

[edit] Types

There are many different types of white blood cells. One primary technique to classify them is to look for the presence of granules, which produces the categories "granulocytes" and "agranulocytes".

• Granulocytes: As Granular Leucocytes and polymorphonuclear leucocytes (informally, "PMNS" or "Polys"), 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. These granules are related to lysosomes found in some regular cells and primarily act in the digestion of engulfed invaders. There are three types of granulocytes: neutrophils, basophils, and eosinophils (named according to their staining properties).

• Agranulocytes: Agranulocytes are a category of white blood cells which are characterized by the absence of granules in their cytoplasm. There are two types of agranulocytes: lymphocytes and monocytes.

These categories can be further broken down as follows:

Type	Image	Diagram	Approx. % in humans	Description
Neutrophil			65%	Neutrophils deal with defense against bacterial infection and other very small inflammatory processes and are usually first responders to bacterial infection; their activity and death in large numbers forms pus.
Eosinophil			4%	Eosinophils primarily deal with parasitic infections and an increase in them may indicate such.
Basophil			<1%	Basophils are chiefly responsible for allergic and antigen response by releasing the chemical histamine causing inflammation.
Lymphocyte			25%	Lymphocytes are much more common in the lymphatic system. The blood has three types of lymphocytes: B cells: B cells make antibodies that bind to pathogens to enable their destruction. (B cells not only make antibodies that bind to pathogens, but after an attack, some B cells will retain the ability to produce an antibody to serve as a 'memory' system.) T cells: CD4+ (helper) T cells co-ordinate the immune response (they become defective in an HIV infection) and are important for defence against intracellular bacteria. CD8+ (cytotoxic) T cells are able to kill virus-infected cells. γδ T cells possess an alternative T cell receptor as opposed to CD4+ and CD8+ αβ T cells and share characteristics of helper T cells, cytotoxic T cells and natural killer cells. Natural killer cells: Natural killer (NK) cells are able to kill cells of the body which are displaying a signal to kill them, as they have been infected by a virus or have become cancerous.
Monocyte			6%	Monocytes share the "vacuum cleaner" (phagocytosis) 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 recognized again and killed, or so that an antibody response may be mounted.
Macrophage			(see above)	Monocytes are also known as macrophages after they migrate from the bloodstream and enter tissue.

[edit] Medications Causing Leukopenia

It is well known that a variety of medications can have an impact on the number and function of white blood cells. The overall white cell count may be reduced in leukopenia. Alternatively, there may be a reduction in one of the populations of white blood cells. For example, if the number of neutrophils is low, the condition is known as neutropenia. Likewise, low lymphocyte levels are termed lymphopenia. There are many medications which can cause a leukopenia. Clozapine, an antipsychotic medication, has a rare adverse effect leading to a total absence of all granulocytes (neutrophils, basophils, eosinophils). Some medications are used for immunosuppression. These include sirolimus, mycophenolate mofetil, tacrolimus, and cyclosporine.

[edit] Other tissue cells

• Histiocytes, found in the lymphatic system and other body tissues, but not normally in blood:
  • Dendritic cells
• Mast cells
• Allergies can also change your white blood cell count.

[edit] Additional image(s)

Blood cell lineage

[edit] See also

• Leukoreduction
• Lymphadenitis
• Phagocytosis

[edit] References

[edit] External links

• Dorlands/Elsevier l_07/12485996
• Dictionary at eMedicine leukocyte
• MeSH Leukocytes