White blood cell

The blood contains erythrocytes (red blood cells), leukocytes (white blood
cells), platelets and blood plasma.

White blood cells 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.

White blood cells are also called leukocytes or immune cells. 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.

A type of cancer in which white blood cells multiply out of control is
called leukemia, of which there are several types.

There are many types of white blood cell:

Granulocytes

There are three types of granulocytes: neutrophils, basophils and
eosinophils, named according to their staining properties.

   * Neutrophils, so-called because they stain neutral, contain multilobed
     nuclei, normally 2-5 lobes. They are the primary cell of the acute
     inflammatory response, and act by phagocytosing foreign material and
     pathogens. Neutrophils are like the 'vacuum cleaners' of the immune
     system, engulfing and destroying pathogens, or tissue debris after
     injury.
   * Eosinophils, so-called because they stain brick-red with the more
     acidic stain, known as eosin. These cells have a role in the allergic
     response, and in defending against parasites such as worms.
   * Basophils, so-called because they stain dark-blue with the more basic
     stain. These cells are the least common circulating white blood cell,
     and their purpose is not that clear. Though it is thought that these
     cells become tissue mast cells.

Other White Cells

   * Lymphocytes, much more common in the lymphatic system, and including
     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 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.

Other Tissue Cells

   * Histiocytes, found in the lymphatic system and other body tissues, but
     not normally in blood:
        o Macrophages
        o Dendritic cells
   * Mast cells