Leukocyte

A scanning electron microscope image of normal circulating human blood. In addition to the irregularly shaped leukocytes, both red blood cells and many small disc-shaped platelets are visible

Leukocytes or white blood cells (also written leucocytes, white cell, and white blood corpuscle) are any of several types of blood cells that have a nucleus and cytoplasm and are part of the immune system, defending the body against both infectious disease and foreign materials. Leukocytes are found in tissues throughout the body, including being transported through the blood and lymphatic system. They separate into a white layer when whole blood is centrifuged.

The main categories of leukocytes are granulocytes (neutrophils, basophils, eosinophils), lymphocytes (B cells, T cells, natural killer cells), and monocytes (including macrophages). These carry out a wide variety of protective functions, including producing antibodies, engulfing bacteria and protozoa, ingesting dead and infected cells and debris, and so forth. However, all of these types are produced and derived from a pluripotent cell—a cell that has the potential to differentiate into a variety of cells—in the bone marrow known as a hematopoietic stem cell.

The number of leukocytes in the blood is often an indicator of disease. in the human body, there are normally between 4×10⁹ and 1.1×10¹⁰ white blood cells in a liter 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.

Types

While red blood cells and platelets (small, detached cell fragments important in repair breaches and blood clotting) are similar to one another, there are many distinct types of leukocytes (Alberts et al. 1989). Three main categories are traditionally recognized on the basis of their appearance under the light microscope: granulocytes, lymphocytes, and monocytes (Towle 1989).

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: 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: leukocytes characterized by the absence of granules in their cytoplasm. These include lymphocytes, monocytes, and macrophages.

The functions and morphology of these cells are 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 and are important in the defence against intracellular bacteria. CD8+ cytotoxic T cells are able to kill virus-infected and tumor 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 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 able to develop into the professional phagocytosing macrophage cell after they migrate from the bloodstream into the tissue and undergo differentiation.

Blood cell lineage

Etymology

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. 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.

Medications causing leukopenia

Some medications can have an impact on the number and function of white blood cells. Leukopenia is the reduction in the number of white blood cells, which may affect the overall white cell count or one of the specific 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. Medications which can cause leukopenia include clozapine, an antipsychotic medication with a rare adverse effect leading to the total absence of all granulocytes (neutrophils, basophils, eosinophils). Other medications include immunosuppressive drugs, such as sirolimus, mycophenolate mofetil, tacrolimus, and cyclosporine.

Fixed leukocytes

Some leukocytes migrate into the tissues of the body to take up a permanent residence at that location rather than remaining in the blood. Often these cells have specific names depending upon which tissue they settle in, such as fixed macrophages in the liver which become known as Kupffer cells. These cells still serve a role in the immune system.

• Histiocytes
• Dendritic cells
• Mast cells

See also

• Leukoreduction
• Lymphadenitis
• Phagocytosis

External links

• Template:Dorlands
• Template:EMedicineDictionary
• MeSH Leukocytes

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