White blood cells
(WBC's), or leukocytes (also spelled "leucocytes"), are
cells of the immune system involved in defending the
body against both infectious disease and foreign
materials. Five 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 WBC's in the blood is often an
indicator of disease. There are normally between 4×109
and 1.1×1010 white blood cells in a litre of blood,
making up approximately 1% of blood in a healthy adult.
An increase in the number of leukocytes over the upper
limits is called leukocytosis, and a decrease below the
lower limit is called leukopenia. 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.
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, typically white layer of nucleated cells
between the sedimented red blood cells and the blood
plasma.
There are several different types of
white blood cells. They all have many things in common,
but are all distinct in form and function. A major
distinguishing feature of some leukocytes is the
presence of granules; white blood cells are often
characterized as granulocytes, or agranulocytes.
There are three types of granulocytes: neutrophils,
basophils, and eosinophils, which are named according to
their staining properties.
There are three types
of agranulocytes: lymphocytes, monocytes, and
macrophages.
Information
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